This hits very close to home for me.
I was assaulted two years ago. When DH and I told my parents we were engaged some time later, they insisted that I invite the responsible parties to the wedding. That was, I'm sure you'll understand, not at all negotiable. But, from their point of view, it would look bad (small town and all) if these people weren't there due to the close family relationship they had.
This was one of the last few straws that lead to a cut. To say the process was unpleasant would be a horrendous understatement. I had counselling for almost a year.
I understand how sticky and tense this situation must feel, and how it can be awful to accept that maybe, no matter what you do, someone will be upset. If that is how it turns out, then please just remember: this is your engagement. Of all the events in life, this is one of the few where you don't have to feel guilty about not doing something that makes you understandably uncomfortable.
Hortense understands how her husband is. Depending on how close you are to her, could you ask her in person if she'd be offended or unable to attend if you didn't invite lecherous Hector as well as her? If she was fine with it, then you'd make your parents and yourself happy.
If your relationship isn't one where this would be feasible, then don't invite them. Part of being a unit with someone else is taking what comes with it. "For better or worse" and all that, if you like. I'm sure Hortense is lovely, but you won't be able to relax during your own celebration if Hector comes along and leers or makes sexual passes at children.
Moreover: the whole point of being a good hostess is doing what is best for your guests. Inviting Hector hardly fits the bill here. It would be a far better thing, hostess-wise, to not invite someone who will upset half the guests and make disgusting, sexual comments at young girls who should never, ever be subjected to such a thing. Allowing this would not be the mark of a good hostess at all.
If somehow not inviting the couple isn't possible either, then you might have to scale back the event, sadly. Unless you feel that having Hector harass your guests is a better option, which I don't.