No, it’s not remotely immature to find sex and sexual compatibility to be an important part of a long-term adult relationship. We’re all different and it’s not ‘immature’ or ‘unreasonable’ to include sex, and an alignment of sexual attitudes, as one of your priorities when looking for a long-term partner.
If a man wanted to wait for months before having sex it would indicate that he and I had very different attitudes to sex and, in all probability, mismatched sex drives. I’m fully aware that some people see sex as a special thing that they ‘give’ to someone only when they are fully certain it is going to secure a commitment, and that’s fine; they can feel that way.
But I am not one of those people and I would not be compatible with someone who is. I would not be attracted to someone who felt that way about sex. That doesn’t make me immature. It makes me pragmatic about what I need.
And, as I’ve been with my lovely, kind, clever, funny partner for 23 years now, it’s worked out very well for me. (The one relationship I’ve had that was truly miserable and damaging was the only one where I told myself that his different attitude to sex shouldn’t matter because it was ‘only sex’, in fact!)
There are million threads on Mumsnet where people are really struggling in their relationships and suffering significant upset because they and their partner have completely different needs and attitudes towards sex. It isn’t immature or unreasonable to want to avoid that sort of heartache by looking for a partner who is on the same page as you from the start.