I feel like there is a difference between finding someone attractive enough to entertain the idea of having sex with them and starting/maintaining a friendship for the sole purpose of keeping open the possibility of sex in the future when they can exploit a vulnerability. If a friend treats everyone of his/her friends roughly equal, I see no harm. However, „friends“ that bend over backwards, get overly touchy and so on with the people in a committed relationship they wanna bang and not with people they don’t, I see a problem. Worst case yet that i encountered was a „friend“ that mimicked mannerisms, hobbies and appearances of the partner that „stood in his way“. That’s just plain creepy.
