Whether blast & cruise or cycle & PCT, when you're "on", your endocrine system is "off". There's no "reducing fertility" by using ancillaries while on the cycle because your testicles will still stay "off". You're either "on" or "off", where it's like a light switch. So just as you said in the first two sentences, if you want to have a kid you'll ideally have to come off completely and run a very strong PCT until your lady gets pregnant. But that said, there are many people, some I know personally whom have had kids while blasting so it really is a subjective issue here.