They're all victims, he wasn't into any of them. And when he could still be bothered to choose, he says he tried to choose "beautiful people", not "beautiful men." But it's a fact that men would be easier to pick up the way he did was doing it.
As for remembering 2 out of 1000 victims, he remembers more. When you find them in Cazador's basement he says he remembers all of those faces. We concentrate on Sebastian because Sebastian is the one who talks to us. The actual problem here is that Cazador's mansion in half assed and the devs just threw a few npc models into one little cell and called it a 1000 spawn.
Bi and pan also isn't 50/50 irl. Having some preference for men doesn't mean he's meant to not want women.
He says "beautiful people" it is one of the excuses I used to make myself

. And he says he remembers many of these faces, not all.
My point is exactly this, Larian could've made Sebastián a woman, because there was already a man in Astarion's past. But they didn't, did they. They could've made an unknown female to carry the conversation, but they didn't. There is no variation in Astarion's past lovers that HE himself tells you about, they are all men. It is in the game. I don't need to make things up, Larian made it for us, I'm just stating what is in the game.
I'm also not sure, what do you mean by it was easier for him to pick up? If anything I'd say it would be easier to pick up a female, than a male just based on a real world statistics. The vast majority is hetero and I dont see how any straight man wouldn't tell him to get lost right off the bat

.