Most (so, not all) ovoviviparous livebearer females are able to store sperm packets in the folds of their fallopian tube for over year even. Mollies and guppies belong to this group.
Can mollies crossbreed with guppies? Yes, they can. But if both genders of both species are in one tank, the females prefer males of their own kind as a mating partner despite of the long chasing behavior of the males of the other species. Would you keep only the opposite sex of of both species in one tank, it's more likely that a female gives in to let a male of the other species mate with her. Hybrids between these two species "can" be less fertile till even sterile.
Even the phenotype of a cross between a molly and guppy differ. For that depends on the phenotypes of both species used. Mollies and guppies can mate because their sexual organs are compatible. So, it's not specifically that they share the same genus. The lower fertility and infertility has got more to do with how close or how far both species are related.
Here's a link:
http://emeraldking-aquatics.com/livebearer6.
A good example that sharing the same genus isn't evidence of actual offspring, is shown in the comma swordtail (Xiphophorus signum). These won't crossbreed with othe swordtails or platies. Their reproduction system is different from the others. But yes, most related species where the sexual organs are compatible with another, can establish offspring (fertile, less fertile till sterile).