Sex on first date? Does it really matter if it happens on first, second or third date?

So I started dating after taking a break after a long term relationship. One time a friend (guy+girl) said "Well you can't expect to get serious with someone, if you sleep with him on the first date... you at least have to wait 3 dates." But then I am asking myself: if someone just wants sex from you, isn't he also willing to wait 3 dates, then sleep with you and then let you down? I don't get this whole thing. I mean, I understand that if you are a person, who wants to built a relationship first and then have sex a little further down the road, I get that. But I can't believe that a guy that just wants to sleep with you is not willing to wait another date to do so. You know what I mean?
In general, for me sex can happen early, because for me it is an important part of a relationship.

I also believe that there is a difference between "first date", where you maybe wrote with someone for two days, then go on a date, or if you have been in touch with someone for longer and then go on a "first date", no?

I am asking out of the following situation (and obviously bad experiences haha). I started writing with a guy about 5 months ago (we got in touch on social media due to mutual friends). We did not manage to meet in person till mid January because he lives in another city and I was out of the country for about 6 weeks. We met 1 time, got along reaaally great, talked the whole night and also ended up kissing. After meeting we have been in touch more or less every day but have not seen each other again because he is out of the country. Getting back he will pass through the city I am living in and said that he would like to stay for a couple of days, to get to know me better, hang out etc, if I would like that, too (I definitely would). So now I am obviously thinking about sex with him and wondering if it could screw up the chances of getting into something more serious with him if I sleep with him, lets say, on the first night he gets here.

Opinions? experiences? Thanks!

Sex on first date? Does it really matter if it happens on first, second or third date?
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