dominance or sexual interest?

Junior is a 10mo male who was neutered in September. Missy is a 4mo female who won't be spayed until mid-January (the vet wants to wait until she's closer to 6 months).

In the 2 days they've been together full time, Junior has been obsessed with sniffing her nether regions and twice now I've seen him straddling her. She hasn't given any signs of heat and both times her rear end has been flat on the ground, not up in the air.

My question is: is Junior just asserting his dominance over her or is there a chance he is sexually interested in her even though he's neutered? This morning he's made a little grunt-like call a few times that I've never heard from him before. I have no clue if that signifies anything, though.

Mez

edit: I just realized I posted this in Care & Grooming. I meant to post it in Behavior. Will a mod please move it if necessary?

Neutered animals can and do still perform, they just shoot blanks . It sounds more like sexual interest. Signs of a female in heat are :meowing incessantly, rubbing on stuff, and sometimes humping things (my friend's female cat did that before she was spayed).

Just wanted to mention that she could be in heat, sometimes cats have silent heats and can still get pregnant during the silent heat so make sure she absolutely cannot get outside.