Vermandois is a grey area. Mostly because the "proofs" that he was gay date from after the period. No exact contemporary comments on it- when they certainly mention other gay figures (Monsieur, Le Grand Conti, the Prieur de Vendôme, the chevalier de Lorraine, the duc de Nevers, etc), no mention is made of Vermandois in that list. More than that, Liselotte- who was Vermandois' "guardian" -expresses a concern that he would be removed from her care because of what Lorraine did (i.e. "induct" Vermandois) in one of her letters. Lorraine was known to exploit Monsieur's feelings for him to get away with behaving "far beyond the pale" (both Minette and Liselotte were "victims" of this in that he treated them with absolute disdain). Louis XIV would usually respond by sending Lorraine from court and then Monsieur would whine/cry/huff until Louis allowed Lorraine to come back. Whereupon Minette/Liselotte would whine/cry/huff about it. One of the charges against Lorraine was that he was involved in poisoning Minette, and Liselotte certainly held him in contempt (not for his lifestyle but for his behaviour towards her). She tolerated Monsieur's relationship with him, but confessed that burning their "perfumed correspondence" (after Monsieur died) the smell was enough to make her vomit.
Now, to return to Vermandois, the age at which this "induction" would've incurred is excessively young. And given that there's no further mention of his behaviour (surely if he were gay, the soldiers he fought with in the Low Countries and Hungary IIRC, would've mentioned it. Even as a joke), I tend to think that it was either a once-off, perhaps of dubious consent at best. Or it was what would be defined as a rape of a minor (hence Liselotte's fears that she would be blamed)- as said, Lorraine was a known brute.
Vermandois died too young to know exactly how he would've turned out (pardon the bad pun), but I'm not convinced he fell into the same category. I think it was simply a thing that courtiers perhaps used as a "post facto" proof to justify why the king treated the able-bodied Vermandois with lower regard than his favourite, the handicapped Maine. Maine being treated better than his full brothers was no issue (see César de Vendôme vs his brother), but the favouring of a son born of double adultery over one who didn't have that stain on his cradle...was different. And given his sister's own potential lesbian affair with (think it was the princesse de Monaco- who'd also suggested one with Liselotte as "payback" for Monsieur), plus what had happened with Lorraine, it was pretty much baked into the cake they'd accuse Vermandois of this.
Sorry for the rant