I am with team "Do not invite hector, but invite hortense."
A few things really stuck out to me in your OP.
He makes me feel really gross, in fact. He's been doing so since I was a tween. I do not tolerate his behavior, and I avoid being at events where he is.
Here's the issue: Hortense is close to my mom. I would love to invite Hortense to the party, for my mom's sake. I will not invite Hector because I can't trust for one minute that he won't try peering down some fifteen-year-old's dress or telling my future S-I-L just how lovely she looks in a tone that will make her want to shower with a steel brush and bleach. I will not spend the evening running interference on his lewdness.
Those two comments really stuck out, especially the bolded. He has been making you feel gross since you were a tween. That would be what, eleven, twelve? And not just gross, I'll wager. Maybe he even made you feel unsafe. So you know exactly how your guests are going to feel.
The second thing that stuck out was that second sentence. Peering down a fifteen year old's dress. Fifteen. That means the young lady is still a minor. She might not know how to handle the situation like you can. She might not be used to Hector's ways. And you said he's been making you feel gross since you were a tween. You think you are the only tween he's done that to? There were probably more before that, and there will probably be more after. Protect them. Don't invite this this... person to what is supposed to be a celebration of your upcoming marriage.
If your mother protests, explain what you explained to us. "Mom, Hector has been saying suggestive things and making me feel uncomfortable since I was (insert age here). I am done with dealing with this. He looks down peoples dresses for cripes sake! Some of the people he does that to are minors. Shouldn't we protect them? And what about the other ladies? Don't they have a right to feel and be safe?"
I am sure other ehellions can come up with better wording, but your OP threw up red flags for me. This man looks down people's dresses, and not even the woman he does it to will not call him out on it? Something is very wrong with that.
edited to add a word.