If my BF posted here, he'd say much as a couple others have said: he and I did not meet because he was online looking for a new Girlfriend to entertain himself, but rather met in the course of our daily lives, in a group for an interest we share. We'd known each other a long time before I got divorced. We were friends.
Of the other serious girlfriends he's had in the past, it's been the same. He's just been carrying on with his interests and met them, while they're both doing things they love
FWIW, I would never have given the time of day to someone online or some guy I'd just met who was married and wanted to date me. Yes, I'd think it was sleazy. After BF, I will never, ever do it again. There are some serious flaws inherent to dating a married man. BF himself, in our two years together, has sworn that he will never again date outside the poly world because he has hurt me, he himself feels a lot of guilt, and he doesn't ever want to do this to another person again.