I think I’m going to go with, “the same thing that attracts single men to single women.” The principles of attraction are fairly universal. If they weren’t then you wouldn’t see a power-law distribution to attractiveness. Everybody would be relatively equally attractive.
When a guy looks at a women for the first time, he doesn’t see their marital status, that comes later after he thinks to check for a ring. No, he’s attracted in spite of her relationship status, not because of.