What comes across from what you've said is that your behaviour is governed by what is currently perceived to be appropriate/social acceptable moral boundaries for those who are in committed relationships, while this 'great man' you're living with has no such boundaries.
In essence, he's an egoist for whom flattery is the sincerest form of admiration and he thrives on the admiration of random females as it strokes and sustains his over inflated ego.
Do his texts to you frequently call you 'stunning' and/or 'gorgeous', or are the halcyon early days of unsolicited endearments over?
Imo you're not being at all oversensitive and it's high time you told him to either stop indulging in these self-serving flights of fancy or take a hike. If I were you, he'd be left in no doubt that his choice is my way or the highway. However, that said, any man who was under the impression that he could get away with this type of intensely personal and insulting crap under my nose would find himself on the fast train out of my life in no short order.
Don't be under any illusion, OP; he knows full well that he's overstepping the mark in actively seeking/encouraging women who are prepared to venture into forbidden territory with him, but the irony is that he's not a 'woman's man' and any women fool enough to pander to his ego will be discarded like tissues when the next promising boost to his self-image comes along.
Once the layers begin to be peeled off the onion another irony will be revealed, which is that at heart he's insecure in his masculinity/sex appeal to women and, if she could be arsed, even a novice femme fatale could have him wearing blinders and eating only out of her hand - but why bother to make the effort when so many other horses are willing to lead themselves to water and revel in it?