These are several questions actually.
1. ' does making Ian a manwhore ruin the relationship between him and Lena' a) Technically, the answer is: we dont know that yet, as Lena hasnt found out at this stage of the game. b) There is a strong hint that if she finds out, it might well be Stan denouncing Ian in the near future c) If she does find out, she will most probably take it pretty bad, given the Axel-Cherry antics which are the ONE reason of her breaking up, PLUS when she accepts a serious relationship with ChadIan she DOES warn him their relationship implies him not sleeping around. So I suspect if she finds out about it her reaction is going to be quite somethin.
2. "is it mandatory for lena to date other people or is it possible to make her Ian exclusive" = absolutely. I have one save which I call good girl Lena and she has only sleppt with Ian up to now, girls included. Actually, you could even play the game and prevent Lena to have any sex at all (not sure I'd see the point thu)