Your post sounds really concerning. It sounds like you are " consenting" (not sure it's the right word in this case) to sex acts you do not enjoy our of a desire not to hurt your partner's feelings, or even fear of him? Perhaps fear he will leave you?
It is not ok that you are in this position.
He was completely wrong to start performing an act you had made it clear you did not consent to. It was not up to you to stop him, it was up to him to check you consented, both before and throughout. He clearly did not do this. He is therefore at best foolish thoughtless and ignorant to a dangerous degree. At worst he is a deliberate rapist.
If would be brilliant if you could speak to someone about the relationship and your feelings within it. Maybe a friend? Even better a counsellor or therapist? So that you can clearly work it out what it is you are scared of, and what you really want.
In the meantime, or if you are not willing to do this, do you think you can have a conversation with him about sex in which you reassert boundaries. And agree a safe word. I actually believe this is good in all sexual relationships, but certainly anyone who is experimenting with any kinds of less common sexual practices (yet often they are only used by BDSM afficionados). Or traffic lights - red light/amber light/green light. Green = carry on, I am enjoying it. Amber = this is pushing me close to my limits, please proceed with caution as I am not sure I like this and may stop you soon. Red = absolutely everything stops immediately. (You don't have to go through amber first obviously - both partners can go straight to red at any time). You need safe words and you need to practice using them. If your husband declares a problem with that concept he is an absolutely fucking dick and will be making that patently clear.
As I say, what you really ought to do, I think, is look to leave this relationship. My safe word suggestion would help somewhat in the meantime - that's all.
And your husband's girlfriend he goes off with - are you really ok with that? I know some people are, but it's not all that common. If you truly are, fair play to you both. But often it's a situation women are bullied or shamed into accepting, rather than one of their free choosing.
Good luck OP - please be safe!