The male's penis is a dagger he uses to forcibly inseminate the female, puncturing her body wall and injecting sperm directly into the bloodstream.
So how does he know when he's found a female?
He doesn't.
#DeepDive
See disclaimer:
Male bed bugs didn't seem to be too discriminatory, and would attempt to mount pretty much any well fed insect...male and female.
However, the attempts to mate with males were a bit different than what you'd see with females.
When mounting other males, they'd get right up to the point of insertion...and then quit.
Most bugs can tell the boys from the girls from afar...but bed bugs can't recognize females until they're pretty much touching them.
There's another thing...same-sex mating attempts produced a distinct odor.
Males which can't produce pheromones are mounted longer and more violently.
Male bed bugs aren't the only bed bugs which produce alarm pheromones.
Female bed bugs, and nymphs produce alarm pheromones as well.
The males seem to only react to the alarm pheromones of other males, and nymphs.
Adults produce a mix of (E)-2 Hexenal (think green apple flavoring), and (E)-2-octenal (a mix of citrus and cucumber).
The nymphs produce 4-5 different things, but the main one is (E)-4-oxo-2-hexenal.
Sex is something that's frequently happening in bed bug colonies, and it only happens with living insects. By definition, it only happens in breeding colonies.
If we know what bed bug sex smells like, we can design better diagnostic tests.
Bed bug sex makes a little more sense now.
Insect sex is a bit horrifying, and we should not draw any conclusions about how humans should act from the ways insects do act.
We are different from insects, and they are not role models.