I'm not sold he's in denial about the experiences not being abusive. Because he says the stuff he did at 13-16 were affairs too and the men were in their 40's or 50's I think there's a deeper issue dealing with like trauma... Etc...
I am admittedly NO EXPERT on trauma, but from what I have read most people tend to withdraw sexually. So if you two have been enjoying a very active and fulfilling sex life, it would seem as though your partner grew from those experiences as opposed to shutting himself down. However if he was only 13 and his partners were in their 40's or 50's, then you are very correct to label it as trauma b/c it would be defined legally as "statutory rape" in many cases and his partner should be a registered sex offender.
I am however just trying to point out that regardless of what happened that your partner is somehow managing to have an active and fulfilling sex life with you which based on your description seems as though the relationship with you is a healthy one for him.
we have a great relationship and a great sex life. Plus it doesn't bother me I find gay sex a turn on.
Just do NOT be in a hurry to get married until you get everything sorted out! Take your time, perhaps even go to some marriage counseling together as a preemptive way to help make going into your planned marriage even stronger. Obviously bisexuality will create a unique dynamic in a monogamous relationship over the long term, and you two really should talk about that very openly with a third party that is trained to help couples with things of this very nature. This way you two can get married with confidence and make it more meaningful. Or you may determine that you need a little extra time to help each other sort some things out with one another.
There very well may be some aspects of your personality that would attract you to someone of this nature that you may not fully understand, and your desires for a threesome to watch him be with another man may actually be more traumatic for your partner than you are fully aware. Or perhaps it is not. But I would not get married until you feel confident about everything. Obviously the fact you are here asking for help is a red flag all on its own!
Take your time. There is no hurry. You two will have your whole lives to get to know one another now that you are each working towards getting married. If you do get married, make sure it means something that you can both really be proud to share!