[Suzu]

I love how everyone forgets that one of our most recent riders has a child... Eiji Tomari, anyone?

Really, I generally think that most of our riders can still be biological fathers, as long as they're not completely cyborg or some kind of monsters (that can't reproduce).
As far as I know from the Shin Kamen Rider Spirits manga, Super-1, for example, has almost a full robot body, so he will most likely not be able to reproduce. However, as for most of the others, such as the original Kamen Riders or Amazon, I don't see why they couldn't. Shocker probably didn't have a need to take care of that and I don't think Ichigou and Nigou are complete robots inside, so yeah... should work.

As for the Heisei Riders (might not be completely spoiler-free), Kuuga is kinda out in the open, that really depends on how much the Arcle influences his body, but I do think it might be possible.
Faiz riders can't reproduce for obvious reasons.
In Blade, Mutsuki and Tachibana are fine, Kenzaki and Hajime... massive show spoilers.
Not quite sure about Kabuto because it's been so long. But I do think worms could also reproduce?
With Eiji, it's the same as with Godai, but I do believe the medals' effect wouldn't be as extreme as the Arcle, so I think he should be fine too.
As for Wizard, both Haruto and Nitoh (as well as the Mages, with one of them already having a biological child), are still humans, so no problem.
Gaim... I would say no problem there. Going from mythology, most gods could and would reproduce. A lot.
In Drive, the only exception is of course Chase.
And Ghost... well as Aran does have a father, I assume Ganma can actually reproduce? We still don't know enough about the Ganma.
As for Makoto and Takeru... they still have their physical human bodies so that should work.