Throwing some cents into this:

For all the women out there buying ovulation test kits, having an application that can actually predict (depending if their cycle is a stable one), when they're most likely to get pregnant would be a great idea.
So, target it towards couples who could use an extra pointer in when to try the hardest.

Personally, I would not target such an application towards
1. men worried about PMS (or any other things they really, and I mean, really, don't understand)
2. women who would like to use "natural" birth control (because that's not reliable)

By the way, having worked in health counselling, you'd be surprised how many girls and boys don't even have the slightest idea how their inside work...
I can't even count the number of times I've heard girls ask: "Can I avoid getting pregnant, if I only have unprotected sex on my non-period days?" and others checking to make sure that they never, ever can get pregnant while on their period. (The answer to the latter is, "there are no such thing as safe days")