I think before you put your fingers in you might want to ask. Plus you would probably want to lube your finger(s) so you don't hurt him. Liking anal stimulation doesn't mean your any less straight or more gay. Many females LOVE anal stimulation and it's not because they are lezbians
I don't think it has anything to do with how you identify your sexuality, straight, bi, gay or otherwise. Some gay men don't like anal stimulation at all and MANY straight men LOVE anal stimulation.
Some men do REALLY enjoy penetration. Other men ONLY wish to be touched externally. And some men... don't like it at all! It's "safe" if you don't use a lot of force and wash your hands. Yes you are running the risk of getting "poo" on your finger. AND you should keep that area clean if your playing with it. It can be a sexy spot... but it's still your anus and bacteria still surrounds the area. (I would recommend only after showers, or using a baby wipe to clean the area.) Wash your hands with soap when you have finished. Also, SHORT NAILS, you would not want to insert a fingernail (sharp or long) into the anus. You don't want to cut. Also, I would only start with one finger.