marriage question bible.?

Incest certainly does seem to be a universal taboo and is considered to be wrong among humans, Saruman. Why? I don't know. 

It's true that lots of people will use the Bible as their moral compass... but lots of stuff in the Bible is contradictory. Fanatics will loudly use the verses that they agree with, and quietly ignore the contradictory ones. 

Yes, the Bible does say this:

" ... do not have sexual relations with your sister... "

Leviticus 18:9 

But the Bible also says a bunch of other stuff that is totally insane: 

"... a woman who isn't a virgin on her wedding day shall be stoned to death..."

Deuteronomy 22:20-21

"... whomever blasphemes the name of the LORD shall be put to death; the whole congregation shall stone the blasphemer... "

Leviticus 24:16

"... if a man commits adultery with another man's wife; with the wife of his neighbor, then both the adulterer and the adulteress must be put to death... "

Leviticus 20:10

"... you shall not allow a woman to live who practices sorcery... "

Exodus 22:18

"... anyone who dishonors father or mother must be put to death for they are guilty of a capital offense... "

Leviticus 20:9 

"... you have six days each week for your ordinary work, but the seventh day must be a Sabbath day of complete rest; anyone who works on the Sabbath day must be put to death... "

Exodus 35:2 

"... your male and female slaves shall come from the nations around you; from them you may buy slaves... "

Leviticus 25:44 

"... a woman should learn in quietness and full submission; do not permit a woman to teach or to have authority over a man; she must be silent... "

1 Timothy 2:11-12 

"... if a man lies with a male as with a woman, both of them have committed an abomination; they shall be put to death... "

Leviticus 20:13 

"... if a priest's daughter defiles herself by becoming a prostitute, she also defiles her father's holiness, then she must be burned to death... "

Leviticus 21:9 

"... do not trim off the hair on your temples or trim your beards.... "

Leviticus 19:27 

"... if you hear that in one of your cities... that certain worthless fellows are saying, ‘Let us go and serve other gods’... then you shall surely put the inhabitants of that city to death, devoting it to destruction, all who are in it and all of their animals, with the edge of the sword.... "

Deuteronomy 13:12-15 

Question: If even God manages to get His Word so screwed up, then how in the hell does He expect us to make total sense out of it??? 

Answer: It's essentially impossible. 

Years ago, my sister and I did enjoy doing lots of really fun sexual incest activities with each other in our teens and into our twenties. Am I sorry or do I feel guilty? No; I'd do it all again in a heartbeat. Why? Because I love my sister and still do; she feels the same way. The fact that we were raised as semi-Muslims (we were never serious Muslims; we're not Muslim today) in the Middle East and we had virtually no contact with anyone of the opposite sex who was our age was no doubt a huge contributing factor. 

So... I enjoyed a happily satisfying relationship of intimacy with my sister, and I enjoy looking back on our romance with wonderfully fond memories.

The bible talks about homosexuality and incest being sexual sins and perverted

- Wearing cotton/poly is perverted, too.

But why is same sex marriage and relatives marrying sinful?

- Because 4000 years ago it had to do with health reasons, today, we have functional brain cells.

The Bible means absolutely nothing. The Bible isn't the law of the land. Only Christians obey the Bible. Everyone must obey the law of the land.

Sins harm humans' souls and offend gods. Souls do not exist and there are no gods. Therefore, sin cannot exist.

Being asexual, bisexual, gay, lesbian, straight or transgender is perfectly normal. It is neither sinful (cf. above) nor perverse. The marriage of same sex couples is a human right.

Incest is wrong because it introduces the many emotional problems that surround sex into already complex relationships. It could permit abuse of family members leaving those in the wrong with the potential defence of this is normal and acceptable. It greatly increases the chances of babies being born with significant and complex genetic disorders.

Same sex marriage is perfectly OK and does not need to be sexless.

Many state jurisdictions and the laws of some religions prohibit, sensibly so, people too closely related from marriage. Unfortunately, not all religions hence the significantly above average numbers of children with genetic disorders in some Jewish and Muslim communities.

Actually, a completely sexless marriage is not a true marriage. This isn't just a religious teaching - even civil law recognises that a marriage which is not consummated by sex is invalid and can be declared nul and void (I'm talking about English civil law here, but I expect the same would be true in most countries).

Most of the time God does not explain why a particular behavior is a sin, and such is the case here; however, the Bible itself may provide us the material for a reasonable guess. It’s probably safe to assume that the first command God gave mankind was probably fairly important to him. So what was that command? “God blessed them [Adam and Eve]. God said to them, “Be fruitful and multiply ...” (Gen 1:28). It’s impossible for a homosexual relationship to produce offspring, thus impossible to obey God’s first command. Since we are created in God’s image, sometimes we can do a little reverse engineering to figure out God’s nature. It appears that just like parents look forward to their children having children, God appears to be the same way. In fact, most likely we get that desire for offspring from God because we are made in his image.

When it comes to incest, it was only a sin under the old Jewish covenant. Under the new covenant, it is not called a sin, thus is not. My guess as to why is that God wanted to ensure a certain amount of genetic diversity among the ancient Israelites..

Sin is anything that goes against Jehovah's Laws and Principles and High Moral Standards

The Greek New Testament condemns "male temple prostitution" - "homosexual" is a poor translation (the Greek word for it never appears). The former refers to participating in pagan sex rituals which were not necessarily homosexual in nature.

only lawful relationship is between lawfully married husband and wife in order to have kids on non-fasting days.

if kids are conceived on fasting days (like wednesdays and fridays), then kids will be sick.

forgive me.

God created relationships to function in a certain way - the man dominates the woman and takes care of her - the woman submits to the man and sacrifices herself for him. When you go against these norms you are warring against human biology and that makes the relationship less perfect than the relationship God designed human beings to have. It doesn't mean the relationship is completely pointless but its not ideal.