What to do if your partners fantasies freak you out? How to push aside insecurities?

Hey everyone, thanks for all of the great advice you guys give. Sadly, I'm struggling pretty bad with what to do in my relationship situation. As my previous post alluded to, my partner of 6 months recently said in passing to some of our friends that he'd thinks bukkake is hot in a fantasy type of way, and that he'd be "into it". It really caught me off guard as I took it as he wants to actually pursue group stuff. All of this was said while having drinks with some close friends, and he states that he was tipsy and they had been talking about sex for a while before I got there, and that none of it meant anything with any realness, just fantasy hot stuff. Both of us are newly out a year-ish and have been together for 6 months. We are inseparable and I could never imagine being apart or otherwise being more happy with the relationship. He was very, very clear with me that he wanted to be exclusive when we started dating, and he says he's super happy with our relationship and thinks about someday marrying me and that he constantly thinks about settling down and that I'm the one. I certainly thinks he's the guy for me. We have talked about stuff we are into in the bedroom and none of this ever came up as what he wants. I did some reading on the subject and I am sure that I think differently from most men sexually. I don't want one night stands, I don't want other guys. I still definitely fantasize - I even actually think bukkake porn is hot, it's just not something Id ever want to do in a relationship. He said if the tables were flipped and I had made that comment, that he'd think that was a turn on as we would have similar interests, and that because he trusts me so much and is so secure in our relationship, even the thought of me in such a group setting doesn't upset him as he separates sex and love mentally, but that he doesn't want to ever actually see me with other guys.

When we talked last night, he said he was not wanting a threesome or anything group related and that he wants monogamy and marriage with me. Indeed, before we started dating in the summer I remember him telling a friend that he could never do an open relationship, and that if he started to date someone, it would be with someone he could eventually marrying. Both of us kept things casual when we were dating / hanging out for the first while until he stated that he realized that he wanted to settle down with me.

So I guess this all boils down to my fears and insecurities, and Im upset that I cannot help feeling this way.

1) Im so scared that we're sexually incompatible and that he is not being fully honest with me as he fears losing me. He finds group stuff hot. Is that common? I know its common to fantasize about other guys, but saying you'd "be into" bukkake is very extreme fantasy wise isn't it? Do people who never want to do those things fantasize about them and talk about them? He states that it was just something that slipped out while drunk, but to me Im worried that he's polyamorous and doesn't know it yet. He finds group stuff hot and separates sex and love pretty easily, and even the thought of me strictly sexually imagination wise doesn't bother him at all. Is that normal for guys? Can couples get past these things or should I end what is a perfectly amazing relationship with the guy whom I think is my soul mate because Im deeply, deeply afraid that he is so sexually and emotionally different from me.

2) Is my insecurity and jealousy normal? I don't want the mental image of him with other guys. I was totally fine until those images popped in my head and they are messing me up as the thoughts are so uncomfortable. Its fine to fantasize, but if the fantasies don't involve me, theres nothing I can do. Im so afraid that he hasn't explored enough if he's having group fantasies and that two or three years down the line he will be sexually miserable with me. I also want someone who wants only me. Want the idea that my partner sees me as an exclusive prize and not some whore. Is it normal for guys to not be bothered bimetal images of their partner with another guy? he states its all positive stuff because he loves and trusts me so much that those images don't bother him.

3) Is he polyamorous and just doesn't know it? I am so new to relationships that I had never thought about this sort of stuff? Its so far from my realm of thinking that I strongly worry we are completely incompatible sexually. He states he wants monogamy and me, and in the least is willing to sacrifice his fantasies to be with me. I am actually very open in the bedroom and not close-minded at all for fantasies I can be involved with, but I don't want to date a poly guy - ever.

Where do I go from here. Im so messed up over it. I really believe I may need to break off the relationship over this, but realize Im tired and emotional, and am probably over reacting. It just hit an emotional cord with me completely. I think its all based out of fear, but I feel so insecure that I cannot sexually ever gratify him as he thinks about sucking off a room full of guys, I worry he hasn't sowed his oates completely. Im worried that because he doesn't seem to even be bothered about an imaginary situation in which Im with another guy that he's gonna eventually wanna do that.
What do I do??