Set wrote:
I'm kind of inclined to say no.
AFAIK, summoned creatures can't be milked of venom (or milk, for that matter), for instance, nor do any conditions they suffer while visiting the material plane (such as being poisoned or energy drained or diseased or cursed or hit point damage) seem to affect them if they are resummoned a few rounds later (the Eidolon being a fine example, as the Summoner class would suck if the Eidolon retained any disease, poison, curse, ability drain, negative energy levels, etc. inflicted during previous visits! 'Hey, we fought a wight, and now my defining class ability is permanently weaker than it was!'), so I'd say no to pregnancy (or parasitic infestation or gaining the ghoul or vampire templates or whatever).
While there don't seem to be any hard and fast rules, I think of summoned creatures (as opposed to *called* creatures) as being like the fake bodies created by astral projectors. If they get messed up or killed or even buffed or enhanced, those changes don't carry over to the original creature (assuming there even is an original creature, and the summoned creature isn't just a temporary manifestation created off of a template and cease to exist the moment the spell ends...).
By the same logic, a summoned creature wouldn't be able to impregnate anyone either. Woo hoo! Safe sex, summoner style!
Ah, but can't they? A summoned spider has venom just like a real spider. They poison your enemies after all. Is there anything stopping you from milking said spider? It has the same abilities, after all. It seems to me that the main reason such thing is not done usually is the duration, as it's not very profitable to gather venom if it will just disappear half a minute later.
However- A level 20 Occultist, an Antipaladin, or to a lesser (ironically) extent a Summoner can circumvent this as their Summoned Creatures are of permanent duration. If you command a Succubus to grant you a profane gift, this profane gift lasts until the Succubus is dismissed, violently or otherwise, right? But if the Succubus is never banished, you'd have it permanently. Similarly, an eidolon will remain diseased or otherwise hurt until it is dismissed. If they weren't dismissed at all, they'd have to recover naturally, right?