Is it cheating...?

So my boyfriend and I have been together for 2 years. We have a good relationship and we love each other so much. Recently he told me that he has some fantasies he wants to make true. He wants to have a threesome. Also he wants to one day try being with the same sex. I catch him on Craigslist messaging people asking for them to come “hangout” with us. One time I even found a same sex message... he hides with his phone and he’s constantly getting replies, but to my knowledge nothing has happened. he apologized for it when I called him out on it. My question to you ladies is... would you consider it cheating? Idk where else to go to ask... no one knows but him and I but it’s starting to make me think he’s going to cheat on me. My brother died a couple months ago and I’ve been really depressed about it still. I feel like my sadness and being a hot mess will make him actually reach out and make him do this.. I could just be overthinking it but I just dont know... I don’t even know if this is the right place to ask but idk where else to ask...any thoughts? I could use an opinion of someone I don’t know..