My sister was making out/fooling around with her bf and he kinda pulled the blankets over her head, but then she heard his popsocket open. She came out from under the blankets and he had his phone out. She wanted to know what he was looking at, he wouldn't tell her (at first) and the mood was killed. She rolled over to go to bed. He asked what was wrong and she asked again and he was looking at. He's cheated on an ex before, (kind of on a technicality, but that's another story.) So she was worried it was something to do with an ex. He finally shows her and it's girl on girl porn. 

It's one thing to check your phone in that situation but to look at porn is another. My sister is young and can't figure out if this is something to be upset about or not. (I think she's upset but she's kind of naive and wants to see the best in people.)

WIBTA if my sister was upset? 