Fiancé Acted Inappropriately at a Party and I don’t know what to do

Hi everyone. I (24F) went to a Halloween party with my fiancé (24M) at our mutual friend’s house. In attendance was our friend’s partner, my future SIL and her husband, another couple, and some of their work friends.

We were all having a really great evening. No one was too crazy and the vibe was fun and chill for most of the night. When my future SIL and I were ready to go, my fiancé decided he was going to stay because the men were going to play games. Fine.

We get back to SIL’s house where fiancé and I were going to stay the night and we continue to talk and hang out. A little while later she gets a phone call from our friend, the host, and he says that my fiancé needs to leave because he was acting inappropriately and had become belligerently drunk.

He proceeds to tell SIL that my fiancé was touching other women at the party inappropriately and kept repeating the phrases that “he thinks (my name) is still here” “he’s so hammered that he’s confused” and “he needs to leave”. At this point, all I see is red. SIL is trying to keep me calm before she goes to retrieve my fiancé. When she brought him home, he was stumbling and saying incoherent gibberish. I removed myself from the room, and this morning I have returned back to our shared home. He is still at SIL’s house. SIL has broken the news to him of what exactly he did

SIL is being a supportive angel, but I don’t know what to do. This situation is wrong on so very many levels. I feel like everything has come crashing down around me. We already have our wedding venue/date, my mom has just dropped a pretty penny on my dress, and I have no support system outside of my SIL right now. Any advice would be appreciated; thank you in advance.

Also I’m posting on mobile, so I apologize if the formatting of this is all wonky.

Edit: For clarification, the aforementioned touching was grabbing of the waist to two different women who both had partners in attendance. The host genuinely believes that fiancé was obliterated and confused (fiancé apparently did not remember SIL and I leaving). Also, to answer one of the most repeated question in the comments, this is completely out of character for him as he has never acted like this before when alcohol is involved. Fiancé’s drinking habits are a couple of beers now and again, but we rarely drink to the point of drunkenness anymore. In the past when we have partied hard, he has never acted inappropriately to anyone else or myself. I wasn’t monitoring his consumption because I didn’t really think that I had to.

Also mini-update: I have taken the initiative to find a couples therapist for us both to at least navigate this incident. I have started looking for an individual therapist for myself, too.