Whether you follow the inane and archaic “third date rule” of having sex or follow your own path (be it having sex on the first night or not for a month, etc.) sex can interfere with common sense. It doesn’t matter if you wait one day or one year, sex will always be a powerful drug. But, having sex too quickly is much more dangerous than having spent any time at all getting to know someone. When the sex is good, when it’s great, when it’s the best sex of your life… you will let other not so good/great/best attributes about your partner slide.
-Sex is phenomenal but he/she has no motivation in life to become anything other than an entry-level employee? Oh, that’s ok, I can live a simple life on a conservative budget. Would you ever accept that otherwise?
-Sex is amazing but he/she is still attached to their umbilical cord and can’t make any life decisions sans parents. Eh, no biggie, now that I’m here he/she will grow up. If they haven’t flown the coop by now, what makes you think you will have any influence?
-Sex is earth-shatteringly good but he/she isn’t Jewish. No problem, I’ll just sacrifice everything I’ve ever wanted because I can live without a Yid for sex like this. What happens when the sex starts to get commonplace?
The longer you wait and the more you get to know someone and build a foundation before having sex, the better. Sometimes people who have sex on a first date live happily ever after and sometimes a simple life is okay and sometimes people cut the cord and sometimes interfaith marriages work… but do you want to rely on “sometimes?” When good sex is the only thing you have in common, common sense ceases to exist.
Appreciate good sex for being just that but don’t kid yourself about turning it into something that it’s not when your partner doesn’t have any of your top preferences or share any of the same values.