Oh yes I think they see Jack as shagging men, it's just that idea of romantic love being between a man and a woman that I think comes to play. I think if both J/I were gay, it wouldn't be an issue and people would see it as a "proper" relationship, but the bisexual thing is often seen as the guy shagging men until he settles down with a woman, which unfortunately is often the case, but not because that's natural, much more because of societal pressure, whereby the the advantages to the man of settling down with a woman/having children win out in the end. The writers don't help at all, with the sentimental side of Jack being about Estelle, and his wife. (Not to mention the further back story we are getting in S3) then he refers to boyfriends when telling sex stories.