So GF's mum is a hypocrite as well as everything else 🙄 She sounds awful.
As someone who was brought up in a very intense and restrictive religion with the same beliefs and sentiments as this one, I would not want my child to be involved in a serious relationship with anyone who had this background. Its incredibly controlling and if this is the way GF's mum is behaving now, I don't think she'll get any better and if he did choose to stay with GF I'd be worried about what his life will be like.
You say that they work well together despite the difference in their religious beliefs, but do they really work that well if he is now being pressured to sit and listen to a sermon from her mum about beliefs that he doesn't share? He respects his GF's beliefs and religion, but she's not respecting his lack of belief if she's forcing him to be part of an uncomfortable and inappropriate conversation with her mother just because she has these beliefs.
I really feel for you as this is wrong on so many levels. I agree you have to be careful about interfering but I also agree with others that if your son is coming to you for advice about this, there's nothing wrong with sharing your concerns re: control, inappropriate behaviour etc and ask him to think about what the future might look like for him if he allows his GF and her mother to basically force him into things like this.
The mother sounds awful and I'd be really concerned about your son being controlled by her, the effect on any future children etc
From personal experience of an organised, restrictive religion I would be telling DS to think very carefully about whether he wants to be involved in this. It might be fine if his GF respected his lack of religion and her mother was not controlling her, or even if she could stand up to her mother and make clear that she cannot ask an adult man to come and be lectured and forced to talk about sex, and that her mother cannot control their relationship...but its really concerning that GF isn't willing to do this.
If GF wants to allow herself to be controlled by her mother (and her religion, because I believe that organised religion is by its nature extremely controlling) then that's her choice, but she cannot put this onto your son and ask him to live that life. I'd be concerned that she is taking her mother's side on this, it indicates that she herself is controlled by the religion and, in turn, willing to let this control her relationship. I would gently encourage DS to think about what this means for him, and any potential children he might have with this girl.
The mother sounds like she could potentially be emotionally harmful to children/young people. If she's anything like my own mother she won't think twice about trying to 'educate' young children about God and what he finds sinful (including reference to sex) and won't hesitate to sit her grandchildren down and have the kind of talk you've described. (Although tbf my own mum didn't go as far as subjecting my DB to things like this, as she realised that he was an adult and it wasn't her place to summon him for a lecture!)
Unless GF is able and willing to put boundaries in place, your DS's life could end up being very difficult. I hope he can find a way though this. Best thing you can do is be there for him and gently encourage him to carefully consider all of the above before making a decision about what to do. It has to be his choice, but you can be there to advise and support. Best of luck OP 