My dogs have been propositioned SO many times it's not funny! Usually it's after they've been speutered so it's a moot point, but Halo got lots of "offers" well before that, starting from when she was way too young to be bred if I were even interested (which I never was). I just told people A: she's too young, and B: I have no interest in becoming a breeder.
I got a lot of "that's a shame" comments about my dogs being speutered, but it never bothered me. Even if you do plan to breed some day, or reserve the right to consider the option, I don't think you need to go into a big long explanation about why not now, or with their particular dog. Keep it simple.
Forever would have been too short
NAFA: FM, MBDCh-S