I tend to think of the D&D sourcebooks more as "tools for the DM to make a game with" rather than a set of complete rules for a game in and of themselves. So I both extend and diverge from them often, but I always try to make sure things in my campaigns work in an internally consistent way. So, it's more that outsiders and their home planes are more heavily and directly influenced by concepts. They're still physically real places, but most of them are so pregnant with aligned particles that they react and interact much more strongly with mental/emotional/magical influence. They also often have somewhat different local physics than the Material Plane. There are exceptions to this, of course, such as conceptuals--creatures that are like elementals, but are representations of abstract ideas rather than physical materials or objects (death elementals like Siwan are more properly categorized as conceptuals, for example). Souls also fall into this gray area, since they can generally only interact physically with lifeforce, but in the outer planes they can often manifest as metaphysical forces interacting with heavily-aligned material (a circumstance commonly referred to as "the afterlife"). Outsider is a vast and varied creature type. As a group, the most they have in common is a general sturdiness and talent for learning, a materialist nature, and being composed at least partially of the essence of the outer planes. Their reproductive methods are also, naturally, just as varied. Beings like mephits reproduce in weird elemental minglings, while salamanders asexually generate larvae. Slaadi have a very weird and convoluted reproductive cycle involving exoparasitism, and modrons are literally assembled in factories then programmed with personalities according to their duties. But angels and demons, for the most part, reproduce very similarly to humanoids, coupling sexually to fertilize eggs (which may be carried internally, as humans do, or laid externally, depending on the kind of female involved). Most angels and fiends (like dragons) are also extremely exofertile, able to cross-reproduce with virtually any other form of life. As you might expect, angels make sure the process itself is as pleasurable as possible for the other party, though their ethical alignment does affect how the relationship goes. Lawful angels like Raviel would be completely sure the other party knew what they were getting into and felt no coersion whatsoever, especially in the case of a male angel impregnating a mortal woman (since the consequences are far more significant for her). Chaotic angels of deities like Kord are more likely to hook up with a bunch of groupies at a feast and leave them each pregnant with a bag of gold on the nightstand, still trying to remember the name of that glowing guy they spent the night with (or in the case of female chaotic angels, leave with fraternal quintuplets from different fathers in their womb, while the mortal dads only get the amazing memories). Fiends, naturally, take the opposite approach: they want their partners to suffer, and so make the process as bad as possible, usually in some way suited to their own particular brand of evil. They might make the act itself feel great, but ensure their partner feels deep shame from it afterwards; or mingle pain and pleasure during it; or make it painful and humiliating, but with an intensely pleasurable ending that leaves their partner craving more, like a drug; or simply provide a mediocre experience that leaves the mortal incubating a horrible extraplanar STD or a half-fiend embryo that'll eventually claw its way out. Sometimes they build torments around the act rather than into it, like trading sexual services for increasing debt or devotion to evil, or using desire to lure mortals away from the things that bring them overall happiness into lives of misery. The dumb and strong ones will usually just force themselves on a victim/partner, often concluding with a violent death, mutilation, and consumption (in that order, if you're lucky). It also depends, again, a lot on the fiend's alignment: devils like very rule-oriented trickery and systemic or institutional abuse, while demons are into one-and-done type deals and go with the flow in the moment.
