So, my husband & I have been together for 16 years. While I do oral on him regularly, he never finished in my mouth. I’ve let him try, but had to spit it out. So recently, he brought up him wanting me to swallow. So, because our marriage has been having some issues, I wanted to try it again, thinking it would make him realize that I want to make him happy. So I did some research, the time came and I did it very successfully. No gagging, no issues 
I was proud and afterwards, said, “like a pro!”. Well, I totally regret doing it at all!! Not even 30 seconds later, we were arguing and have on & off since. Apparently because I handled it so well, I must have been practicing on someone else 
He hasn’t trusted me since we started our relationship 16 years ago, because I cheated on my ex to be with him, so he’s ALWAYS suspicious of everything. He’s insistent that ANY guy would think their wife was cheating, if she didn’t swallow for 16 years, then one day, did it perfectly. I can see his point I guess, but I’m extremely disappointed and hurt. I thought He would be so happy, but instead, he was pretty much mad about it. I’ve talked to him about how bad he made me feel, but he stands by his reaction. I just feel like crap about it.
He needs help with his trust issues. Have you shown him the research you did? Explained that you did the research to fulfill a wish of his? That you did all this for him and not yourself?
If you have done this and he still thinks you cheated or practiced on another then I would suggest couples counseling because something else is plaguing him. I know this send like you are doing a lot for him, and you are … But the counseling is beneficial to you as well. You may just find the answers you are looking for that you all yourself everyday about your relationship.
Yes, he knows I researched and did it for him, he just doesn’t believe it. I’ve tried to get him to go to counseling together or separately many times, but he refuses 
That’s just a bad relationship from the start
Trust is lacking in the relationship and if he’s not willing to work on his issues to save the relationship then get yourself ready to leave. If he’s accusing you of bs now it’s just going to continue our it will always be in the back of his mind.
Trust is too important to put on a back burner.