Even now, as someone who's 27 and looking to start a family before I'm 30, I'm having to be more discerning with age when picking a lifelong partner. My women friends haven't been especially supportive of this strategy, and I can see it when I introduce them to a date a few years younger than them. Call it selfishness, call it pragmatism, but when it comes to the health of my future children, I don't mind being choosy.
I think it’s a cultural thing, and personally I wouldn’t judge two of-age people making their own decisions. Really depends on your group dynamics and groupthink.
Also, not every co-ed friendship is rooted in jealousy or sexual desires.
So you have very few fertile years left into which to fit multiple healthy children. 2.1 is needed to merely sustain a population (with all the medicine of industrialized society to keep them alive). For "wild" humans, that number would be considerably higher.