The biggest concern is first that you didn’t get an STD or pregnant. You don’t want to give an STD to your bf, if you did get one.

Then there is the issue to decide what kind of relationship you want with your bf, who you say you want to marry. Did he know you were seeing this FWB while he was away? Would he have approved of that had he known? Are you okay with yourself for having done that behind his back? Are you going to continue to have that kind of relationship? Is it okay for your bf to do the same? There are open relationships. It’s up to you, but it’s only fair to be up front about it and you both be in agreement on it.

Then there’s the relationship with this FWB. You have confusion that he is just a friend, when you know that you have sex with him. He knows it, you know it, it got fueled with alcohol and ended up in sex. No, he doesn’t really have respect for you and your commitment to your bf. He plied you with alcohol until he got to sex with you, as he wanted. Taking off the condom is really concerning behavior (again, fear of repercussions from that).

When I was a 16 y/o virgin 100 years ago , I was slipped a drug and date raped by a 19 y/o I didn’t even know before that first date. I didn’t say no, but I didn’t say yes. I didn’t understand I was drugged or even that there was such a thing as date rape. I did tell him to put on a condom. I had the presence of mind to think of the protection that would provide from pregnancy and STDs, even though I did not have the presence of mind to start screaming and run from his house.

Today, it seems agreed that it’s rape when you are too incapacitated to use good judgment. You were due to being very drunk, which you did willingly. I agree, you were taken advantage of and you also brought it on yourself. It’s something to further explore in therapy. IDK if you need or want to tell your bf and throw away any chance of a life together. It’s a very personal decision for you. Talk with your therapist more.