I don't see tension between the two. Leora has always been more interested in sex than Paul. Paul could go a couple weeks without sex while at times, we see Leora taking care of business twice a day.
I don't know if its a cultural issue, but rarely have I seen Leora initiate sex with Paul. Paul often will come into the room and say 'he's ready' or just give the go ahead to Leora if they've been hanging around each other for awhile. Sex between the two is usually one-sided with Leora servicing Paul more than the other way around. If Paul was to walk in on her a bit irritated, I think his irritation is more towards guilt for his diminished sex drive than it is for Leora taking care of herself. *I* think most women with Leora's sex drive would have moved on to a guy that can keep up with her.
Someone mentioned (who understands Russian) that they've known each other since childhood. If that's true, there's a bond that exists between the two that her high sex drive and his low libido has overcome. In other words, love conquers all.