The point is that a good and responsible father (like Red seems to be) should feel reluctant when it comes to girl-crushes especially when they involve his daughters. This was pretty well implemented before 0.06, but since 0.07 the game has changed pace a bit.
I do agree with you that this doesn't make for a good porn game, but perhaps it does give a more satisfying (or at least more realistic) experience in the longer run.
Well I think it has to do with the fact that once he gave in to Isabel the brakes fell off. He is now fully corrupted and since they want it and his girlfriend is ok with it he's gonna go all in.
This isn't that unrealistic either. Once people drop whatever moral qualms that have been holding them back they tend to dive straight into the deep end.
An example: I knew some kids that grew up in church with me and did their best to live by its tenants all through HS.
When they got to college and finally gave in to drinking at parties..BAM! They went crazy. Dudes was fucking everything that moved and downing bottles of whiskey.
They became a well known party animals.
This is a typical human pattern of behavior when they stop holding themselves back for some reason. Especially if it involves what they considered taboo.
