Nearly half of those polled have dated more than one person at a time, and a fifth have dated three or more people at the same time. That said, Busby's study built on a bit of earlier research, including one observational study that looked at data from the National Survey of Family Growth. One of the reasons it's hard to determine the best time in a relationship to have sex is because there hasn't been a lot of research tackling that specific question. When is the best time to start being sexually intimate in a relationship, according to science? The list includes getting to know the person, sharing a first kiss, then building up to an expression of commitment. Unfortunately, our culture has this vision of the rapist or the sexual assaulter as being some masked creep hiding in the bushes. But this is a good reminder than any good relationship can and should move at a speed you're comfortable with, all you have to do is talk about it. Well, that "not very into it" should be a big, flashing red light to you. Millenials also wait 48 hours to ask about a second date, while older people wait three days, on average. But the good news, at least if you're a millennial, is that you may be waiting a little less time to see if there will be a second date. In fact, Metts' results suggested that couples who had sex first then said "I love you" after had a negative experience: Probably, you've been drinking, which can affect performance in a number of ways. There are exceptions, of course, but sex on a first date is usually not great. If the chemistry is there, it'll still be there the second and third and fourth time around. Valentine's Day is coming soon, signaling a romantic milestone for many couples.