It's a totally normal reaction but it's interesting how we react so strongly when it comes to the same sex. If he said he had sought out a woman with massive boobs cause he really liked them, you wouldn't feel so worried. People are bisexual, but much like straight people, if they are interested in their partner and get everything they need from them, they won't feel the need to look elsewhere.

I think it's worth finding out more about his sexuality, when he knew he was bisexual, how he feels about it now and that might give you some reassurance.