Hi,
It sounds to me that he is just currious. Most people learn about what they enjoy or what other people enjoy by watching porn or at least learn about sex. He also might be bisexual or bicurious.
You could try asking him about it subtuly, maybe hire some lesbian porn and say you might think it would be exciting for you both to watch it, and later some gay porn and say you were curious and make him watch it with you.
The most important thing is to make him feel comfortable and that you understand, Im sure he still loves you, and I'm pretty sure its just curiousity but the main thing is to be comfortable in your relationship and frank with each other, even if its not direct. Besides what's the point in investing time and energy to a relationship for longer than necessary?
I hope this is helpful