Is a mmf threesome with my husband a bad idea

We both want a 3-some but I'm scared it will ruin my marriage
So my husband and I have been married for about 3 1/2 years and we've grown a lot together in our sexual communication and experimentation. At this Point, he wants to see another "cock in my pussy" (his words) and I'm nervous. I won't lie, the temptation is there. I've always been very sexually oriented and its tempting to think about having a real extra cock instead of using toys. My hang up is that I want to stay "pure" for my marriage and I think that if I go through with it, I would be sacrificing my morals.
For his part, he really wants it and he's trying to comfort me and make me feel better about it. But I just keep going over the "what-ifs" and my anxiety gets the better of me and it all seems wrong. But after a few drinks with my husband, I start getting tempted. And my husband's takes that as a sign that I do want it deep down, but I'm just scared. Which I think is true. I just don't want to ruin my marriage.
Also, for a bit of background. He Had a normal sexual background. You know, banged a handful of girls in high school, had a couple committed relationships, sex in between. I would, say from what he's told me, he was pretty average. Some casual sex, one-time hook-ups, friend with benefits, and a couple faithful monogamous long-term relationships with someone he loved.
Me one the other hand, well let's just say I've had a VERY colorful past. I was sexualized at a very young age and I think it became like a wildfire burning out of control. By the time I finished high school I already had no idea of what my "number" was. I'm estimating about 50. Then after that, everything spiraled out of control. I started stripping at 18 and had plenty of guys to choose from........and I did. I chose a lot. I was like a modern day hippie. Free love, sex, drugs, and rock and roll. I was a free spirit and pretty much let my pussy drive the bus. It was fun and I didn't care what anyone else thought. But from my excursions, I got involved with the wrong people and was pushed into the sex-for-hire business. It's a long story but basically I ended up getting married to a man twice my age that I was scared of and had no love or respect for. It lasted 15 years until he passed. During that time, I birthed 4 children which he used as leverage to keep me there. Anyway, I think I slept with another 300+ guys. Most of them were "Johns" that my "husband" (at the time) set up and brought me to. Some were quick bangs because I wanted to, without HIM arranging or making me do. Anyway, his number is Lee's than 50, and mine is somewhere around 350-400. But that's just an estimate.
So skip forward to now. My current husband, and love of my life, opened up to me completely about his past, and after a bit of coaxing and a lot of reassurance, I finally came clean about my while past. He kind of struggled with it at first but now he's okay with it all and even sometimes turned on about it. Which brings me to the point. This is the first normal relationship I've had and I made a promise to myself that I would hold true to my morals and integrity which means being faithful and honest and monogamous in mind and flesh. So we've come to a point where we're both wanting to explore our sex life. I'm tempted but I'm just nervous I think because of my past and I don't want him to think that I would be like I was in my past and I don't want to ruin our marriage.

Has anyone ever experienced this? Any advice?