I would agree...either he was proud cut, or do you know what age he was gelded or if he was ever used as a stallion prior to gelding?
I have a now gelded stallion (is that an oxymoron
) He's been a gelding for 2 years now, but was used as a stud for two breeding seasons. He still thinks he's a stallion, less the total testosterone aggression. He will protect, herd and chase the other geldings away when the mares are in heat. He will mount and "do" a mare. He is not proud cut. Got both testicles and the epididymus were removed.
So, as for your gelding...they can and some do...if I may be so blunt
masturbate. That mare must be a looker and he must REALLY be in LOVE!