Monogamy always kills houses, and especially when the houses start off being wild and crazy. I won't necessarily completely attribute the staleness of this apartment to Foxy getting into a monogamous relationship but I really don't think it helps things either, but the fact is this apartment is a far cry from how good it was when it first started.
Look at what happening with Misty, Lisa, KPLP, all the other people that have come and gone. Nina and Alan despite all the sex they were having were at almost at the bottom until they introduced Serena.. and then Edda/Joe. Most of the other couples that have started off monogamous are trying to keep interest going by either having friends come over (Ary/George) or experiment with bringing in new people into their bedrooms (Cleo/Axel, Darcie/Stifler). If Foxy isn't going to be willing to have sex with others anymore, that's fine. But they're going to have to throw parties and invite interesting people and hope that their friends can bring some interest back to the apartment. To me, THAT'S what's really killing this place.