I understand why this makes you uncomfortable. I don’t understand why your boyfriend isn’t more responsive to your feelings about it. Surely, there are other ways to handle the situation.
I do sympathize with his mother’s inability to sleep with someone snoring up a storm beside her. But it puzzles me why this family hasn’t thought of other ways to handle it. Has your boyfriend’s father talked to a doctor about his snoring. It could be that he has sleep apnea, for example. If so, it can be treated so the snoring stops. If his snoring isn’t that loud, but his wife is hypersensitive to what he does, can she wear earplugs? Is there room in the house for another bed? How about a futon couch that can be made into a bed for mother, when she has to escape her husband’s snoring? Can the family afford a larger apartment so mother can have her own room to retreat to? All those are practical ideas that I hope have been explored.
If every practical solution is rejected, then I’m concerned that there may be something else going on. You didn’t mention whether there is a cultural expectation which has to be considered or whether it is possible that there are other reasons that his mother doesn’t want to be with her husband at night. If so, those issues need to be dealt with.
Regardless, what concerns me most is that your boyfriend gets “defensive” if you try to talk to him about the situation. If the two of you are serious, he needs to take your feelings seriously. And you need to find ways to approach an issue that doesn’t feel to him like an attack.
You two are in the early stages of a relationship. For it to deepen, you both need to find ways to be comfortable together when talking about uncomfortable concerns. I assure you, this won’t be the only issue where you disagree. Learning how to solve problems together is how two people learn to be a couple.
I imagine, for example, that at some point you’ll need to talk about finances and whether he can continue to support his parents and contribute to the support of a wife and young family. There are probably issues about you that he finds troublesome as well. I hope you will both find the courage and the love to start talking about the things that matter as you move forward in your relationship.
I wish you well.