Is it bad to have sex with someone you just met? ?

Yes. End of story

Your question is fugue, do you mean a one night stand, or you only known them for a short time like a week or two. and looking for a relationship.  I think for many reasons, generally most people want to care about the person they sleep with and want them to care about you.  But I can remember, in my late 20's working for an airline, and hooked up with a few co-workers, or met a woman on road trips.  from time to time. Over 5 years, I hooked up with a dozen maybe one 1or 2 more.  Now in fact I ended up dating 7 for at least 4 months, and 3 of them for over a year. Also at 40, I broke up with my girlfriend I was with just over 3 years.  For the next few months I did not want to start a relationship. I really did not want sex at first, with anyone.  But eventually I did, and was not ready to have a relationship yet. I wanted a friend, maybe a little kissing, but not much more, for a while after my long term break up.   When ever you have sex, what I told my 2 nieces, and nephew, be careful, be responsible, and have sex when you feel you are ready. Not because others say you should, not because you boyfriends says he needs it bad, or if he says, if you loved me, you will sleep with my. Or worse, if you do not sleep with me, I will break up with you. Sex is fun, but really intimate.  There are still some people who think it is wrong to have sex unless you are married. Others think it is sinful to have sex unless you are in a relationship.  To these people, it is bad or wrong.  It is dangerous, physically, some bad guys and girls are out there.  as well risky as someone else said, they could have an STD, and not tell you.  I think, when 2 people decide to sleep together, it is no one else's opinion matters, except the 2 involved.  You will always find some people who will judge you.  As long as you respect yourself and the other person and are honest with them, what you do is up to you.  

Marry and stop that habit. Too much of it leads to frustration.

Yes. It's also a quick way to get an STD. You don't know that person or their sexual history and if you end up with AIDS it won't be anyone's fault but your own because you chose to do that.

People usually feel sick, weird afterwards, left wondering why. Best to wait a few months.

Some women expect a guy to brave up and take control of their sexual needs by holding back on sex until the woman feels ready.Those women who end up having sex straight away will usually blame the guy for making them have sex on the first day instead of waiting until they've dated.

I've never found it to be bad. Although I've usually known my partners for a period of time, there was one girl, my very first, who, although I knew who she was, I had only met her an hour prior to engaging in sex with her. That was over 60 years ago and the thought of her still brings pleasant memories. We had a great time in the bushes at a town park throughout that summer.

Bad to whom? I mean, bad in who's opinion? Who has to think it's 'bad' in order for you to agree with them?

think HIV and Covid...If you continue to indulge in random hookups make it a point to be tested on a regular basis (lust is not healthy..Meeting this person could be dangerous for obvious reasons. I would really be cautious 

If you have ZERO emotional feeling then do what you want, but most people that have sex right away without even getting to know the person FIRST, have miserable lives.