If they were truly in love, then they wanted to be faithful to each other ...
I think the more's of the Whampires -- even whampires in love -- are very different around the issue of being "faithful." It's literally a matter of eating, of survival.
... And though it wouldn't be very difficult to arrange, it would still be a pain to have to find a third every time they wanted to have sex ...
I'm pretty sure it'd be *incredibly* easy for them to set up.
The White Court is chock-full of willing -- in fact,
eager! -- mortal partners. $$$ is not an object for them, either. Set up a an apartment next door with 3-4 clued-in models, and most of the time at least one of them is literally just 30 seconds away.
... They probably did know that this was a solution but didn't want to resort to it.
That seems unlikely, tbh. OTOH, I don't have a "likely" seeming alternative to offer.
...
I don't remember the random jogger ...
Very early-on in... I don't actually remember which story. It's while Thomas is living with Harry.
But: Harry comes home grouchy, and a cute girl comes out of the shower. Harry says something biting, the girl snaps back... then concludes that Harry is just Thomas' (gay) lover, being jealous. She calls Thomas "very naughty" or somesuch.