I don't blame anyone in casual relationships for this..
BUT, if you think you're in a serious relationship and can live and do this (from either side), you just don't love them enough yet. Someones level of love is going to get to the point where they think both people should be entirely happy with just one physical being, then the other one gonna have to make a choice. Agreeing to this is not giving a fukk about y'all relationship enough that you eventually give them a 50/50 chance of leaving you down the road.