Why is it that SEX makes someone think and feel it's a guilty pleasure?

The only reason anybody believes sex to be a guilty pleasure is because religious teachings and viewpoints have condemned it as something to be guilty and ashamed about. It's how many of us are raised based on how our parents treated the topic of sex, nudity, and pleasure. Once you can shed that oppressive mindset and just enjoy sex for the wonderful natural pleasure it is, you'll enjoy it a great deal more.

Since you asked, in my opinion there is nothing wrong, guilty, or shameful about sex or masturbation in any of their forms as long as everyone involved fully desire, enjoys, and consents to it.

Because most religions use sex as a way of making people feel guilty. guilt is a way of exercising control. It's almost impossible to live in Western culture and not get the message that sex is supposed to be a "guilty" pleasure. If you don't set your own definitions in life, you'll just use the default, and the default is often not all that good for you. (Sex SHOULD be something you do responsibly.. but masturbation hurts no one.)

Also, because there's an endorphin rush at climax, when the endorphins wear off there's often a feeling of let-down. that's biological. Calling it "guilt" is assigning a value that is a personal interpretation. Personally, I just feel relaxed...

I think it's because you are either treating the other person as a thing, or the other person is using you. What happens is there is no real love going on between the two people. Most people are only made for one other person, not multiple people. Cause there really is only one other person that can put up with you. Plus most people now a days don't have sex to make babies, they do it just for the pleasure, and then they forget about that person. There's a reason why men and women are made certain ways. The man plants the seeds, the female waters it.

Same thing with masturbation! You are treating your body parts as toys. Sex isn't supposed to be an act of lust, it's supposed to be an act of love. You're telling the other person that you are willing to sacrifice your entire being, and life for them. You're giving yourself to them wholly and completely. Love is not a feeling it's an action.

So my main point is, it's a guilty it feels like a guilty pleasure because you're not supposed to be doing that. It's kind of like duct tape. If you stick it all over the place, and peel it off, it won't stick as much as the first time! It's the same thing with people. If you keep on going from one person to the other, you won't be able to stick to one person. You'll just keep on using, or keep on being used!

EDIT: No religion has condemned sex as being something bad. It's only bad when you do it for lust and outside of marriage. Sex is good and sacred. If it were bad, I don't think God would've given us the body parts to do it. And plus through sex you make another LIFE. So when you have sex in the right context, and with the right intentions, you are open to life. Sex is not bad.

Sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety (each known as a sex)

i don't usual use the word sex i say making love to the person you love. and there's nothing bad about having sex with someone. like porn stars they just enjoy what they do. that's just bad in certain peoples eyes.

Sex is God's gift to 2 people who fall in love with each other.

I think it is beautiful and enjoyable.

If you have sex with someone you truly love (each other), the feeling is much different from casual sex.

if people have sex as a replacement for their masturbation it isn't being honest, it's using each other

It is not a gift for "two people", but a gift from God to be enjoyed only in the marriage covenant. Between a husband and wife, and there is no guilt to be had in that.