Easy. Because men (at some level deep down) feel as though THEIRS is the one. Maybe they don’t think this outwardly, but biologically they’re programmed to spread their seed and multiply. It’s an evolutionary trait for reproducing that makes men think “yeah she’s a lesbian but all she needs is one good dick… my dick… to turn her around”. Some men are able to quiet the voice enough to be reasonable and understand, no… not even my dick… Men aren’t dumb. They just “think with their other head”.