Notice the weasel words from the first guy opposing it: < whine - well it's not the ONLY factor - whine >.
Not only is the live-in boyfriend a reliable marker for child abuse generally and sexual abuse in particular, it's a marker for the worst sort of abuse -- not the "he said, she said" variety, but the "dead children with burns and wounds in places you don't want to talk about" variety.
The cause? I think it's a combination of the alley-cat syndrome in the men -- male cats instinctively slay the children of other competing males -- untempered by any moral or religious training, plus the woman who does not protest because she's afraid the boyfriend will leave her and she has no legal status.
As a side note, the overwhelming majority of perpetrators of domestic violence are shack-up boyfriends. Very few are husbands, except for in the Hispanic and Muslim communities.