I know that there are plenty of reasons to be suspicious of men, especially if you read many of the threads on here, but come on, it must take some mental gymnastics to construe his actions as being weird or dodgy, without any more info than we have.
*It's not weird that neither of them wants surgery to prevent pregnancy, and they prefer to use other means to prevent it.
*They have chosen probably the least effective one, but a poor decision doesn't necessarily indicate a lack of commitment.
*It's not weird that he got to the point in the middle of sex to raise it - when else would it be more in the forefront of his mind? Especially as 1) he'd be actively thinking about what he needs to be doing to keep things safe, and 2) presumably this was a joint decision between them and he's been thinking about how to raise it - it didn't just pop into his head in the moment.
Yes, any of us could think of a hundred scenarios where his intentions might be dodgy, but there's literally no evidence for any of them. Indeed, I could invent a story where he's found out that the OP might be cheating and is worried about catching something from her. Horrible, with absolutely no evidence to think it, right? Yep, but no less likely from the info we have.
The absolute most likely truth is that it's been on his mind for a while, based on the weakness in their current method, and it's "come to a head" during sex and he's finally got to the point of mentioning it. He's suggested an alternative, taking responsibility, and should be somewhat respected for that.