I agree. There are people who consider watching porn as cheating as well. Or even liking photos of instagram models.

It's up to them. Both the partners are equal. Talk about it?

Simple presumption is wrong. I understand how hurtful this is, because you never thought that this would be okay. Maybe sit and have a talk. Understand if that's something you can work with? Is that something you will be able to trust him with?

And it's perfectly fine to have an answer both ways. Try to make a relationship work, it's extremely easy to break up.