Since they are both desexed it isn't very likely to be true sexual behaviour. Since your dogs are very close to each other in age it may very well be a powerstruggle. You might get the play to be more relaxed by giving him more exercise such as walking or running.

I applaud you on the spaying and neutering. We need more pet owners like you.

Although neutering and spaying will not remove all sexual instincts...it does remove the bulk of the "urge." This is far more likely to be mostly a power play. Typically a male will NOT actively challenge a female in the canine world. Some times they do. Also typically the one STARTING the fights is NOT the alpha dog. A true Alpha rules by simply being alpha, they rarely show overt aggression towards other dogs. Generally they can bring "rude" behaviors within the group to a quick halt with simply a "look," "oh knock it off, you nitwits!" It's the lower dog that is continuously pushing to show that they are "in charge." In order for YOU to re-establish yourself with your male as being the final word on who exactly is ALPHA, start adding some fun training to his life. Give him something to do, that he can become successful at doing. Basic commands that you'd like them both to be more up and coming in their responses when asked to DO them. Extra fun tricks, such as FIND IT...it may be something or someone. If you teach your dogs to DO THINGS, both useful and fun, you have provided them with ACCEPTABLE BEHAVIORS to do when they start doing behaviors you do not find acceptable!

Dogs reach adolecense at about 6 months when usually neutered. Pure myth that neutering stops male sexual behaviour. It reduces drive but not complete desire or ability. Males still get erections and can perform. Totally harmless as no sperm means no puppies. Now a bit more. Females produce male hormone testosterone too. Take away gonads, male or female and the pitituary gland commands the adreneal gland to take over and produce testosterone. Less than otherwise, but still plenty. Remember stories about harems and eunuchs. Well, eunuchs could have sex with the guy's wives and concubines. But they didn't really care because they could not produce a false heir. Actually, it made eunuchs good lovers because everything slowed. Eunuchs did not ejaculate so it wasn't a 5 minute affair. Course if castrated prior to pueberty another matter. Erections will not occur, little or no body hair, they'll stay soprano in voice, they never get the muscle growth. Got to add the one benefit though. No male baldness.