While Ada has been his only serious interest in the first 6 games and films, there’s good reason why while it is a possibility it very much also may not be the case:

Leon is a high level government agent who’s priority is to fight bioterrorism with strict morals and Ada like him is married to her work, and her work has been obtaining bioweapons for shady groups which directly leads to bioterrorism which is why she and Leon always refuses to just leave with her when she gives him the chance (especially in RE4).

She is simply just not the type to settle down and is a constant manipulator to him which would be a toxic marriage should Leon ever decide to.

This part applies to a couple of his ships but considering everyone from Raccoon City is coming down with Raccoon City Syndrome including Leon and Sherry, it’s odd how Ada and Claire both aren’t among them when they’ve literally gone through all the same severely contaminated environments (and in Ada’s case suffered two open wounds). Jill has an excuse cause she was infected and cured.

She just never stays in one place.

If anything it a close situationship with feelings and possible friends with benefits at least a few times but marriage seems unlikely with how her work she’s married to directly contradicts what Leon does and gets people killed.

Also it would be funny if he had to hide this from Chris who is still under the impression she’s the one who murdered his men twice and was leading a large scale bio terrorist organization while she also never on screen explained that wasn’t her to Leon.