The ex-boyfriend says, 'it's time for me to grow up ; I want kids'

I agree with you, taopolis.
I think the reason for this show is to make gay adoption etc. seem more acceptable and I am completely in favour of that.

I'm not gay but I never plan on having children. I think it's highly insulting to equate procreation with emotional maturity. Anybody (fertile) can have a child, and there are many who have one accidentally long before they are emotionally ready or mature in any sense.
There are a million other ways of being a 'valid' individual (whatever that means) without having children.

I think you are taking in exactly in reverse way .It may be due to sequencing of sentences .

I think they wanted to infer."I want kids ,it's time for me to grow up.
i.e If Monty wants to have kids then his wild playboy lifestyle has to be changed and he has to be more matured and grown up .Monty certainly was not very mature to have kids.

Writer never inferred (at least directly) that only way to be mature is to have kids

I'll agree with you. I know a couple of "adults" (at least age wise: 30s & 40s) who are not mature at all despite having a child. They would rather than play with their XXXXed X-Box or Nintendo than pay attention to their child.

It takes more than having children to be mature.

Children know that magic
makes the world go 'round.
Qqumba Zoo