As others have said it depends on the breed,although it is unlikely she would get pregnant it's not impossible as i myself know of a 5 1/2mth old male getting a girl pregnant. Also it could be stressing your girl as well as him getting frustrated which could and usually does lead to other problems, spraying and aggravation, which would not be good for you or them as it could damage/change a really good tempered cat into an aggressive cat brought about through fear,annoyance, bullying etc. So if it were me i would definitely have them neutered
