I think a better way to ask to question would be "which Champions are promiscuous, which have only been in romantic relationships and which are inexperienced?" Because I do think there is value in pondering something like this; relationships are key to understanding characters and that includes romantic and sexual relationships, too. Someone like Taliyah who is young and probably inexperienced might be taken in by someone quite easily. And then, someone like Darius--who we know was in a likely sexual relationship (I'm assuming that girl was his daughter)--was still willing to kill his former lover, despite probably being in love with her at some point, possibly still, which can speak volumes about his character. The relationship between GP and Illaoi added a lot of tension and realism to their stories: it provided insight into what kind of people Illaoi admires and what she is willing to do despite this, let us see who Gangplank WAS and how he has gone downhill, made it much more emotional for GP to be tested, and more.
So, I don't think the subject is inherently bad. If the humans of Runeterra are similar enough to us irl humans, then their sexual and romantic encounters can shape them a lot, just as their platonic ones can.
Honestly, I think the interesting part of this discussion is that its odd that League has so few confirmed intimate relationships. Obviously, for many the subject just doesn't come up, but the fact that we have to suss it out like this is a little odd consider the majority of the cast is made of human adults. Pretty much only Champions with kids (very few) and GP/Illaoi are for sure, imo. I suppose married/engaged champs could be likely too, but those are few as well. There's 144 Champions and I would say about ~12 Champions that are confirmed in likely sexual relationships is kinda low.
Here's my own tentative personal list of "most likely confirmed to be in intimate relationship at some point" where I got the 12 count, for those interested:
and big maybes to
(was engaged)
(had a relationship with some guy, plus has seduced many)
(formerly married, now unclear in his most recent bio).