How can an aristocracy be perpetuated between creatures that can’t interbreed?

The 25-year-old human prince of a 50-year-old small nation (total population maybe 100,000 spread out in a rather large valley among settlements no larger than 5,000 creatures each) is looking to secure the goodwill of his neighbors. Marrying into a local family or tribe is the plan, but producing offspring to perpetuate the monarchy—therefore the alliance—is necessary for the kingdom’s safety.

However, the creatures nearby can’t typically produce offspring with humans. These include…

  • a band of bugbears (MM 29),
  • a band of halflings (MM 149-50),
  • a clan of dwarves (MM 91-3),
  • a cluster of beholders (MM 26-7) and their slaves,
  • a colony of aranea (MM 15),
  • a covey of green hags (MM 143-4) and its giant and ogre minions,
  • a flock of cloakers (MM 36),
  • a tribe of centaurs (MM 32),
  • a tribe of locathah (MM 169-70), and
  • a tribe of pixies (MM 236).

plus assorted other high fantasy power groups. (Yeah, I know—looking at the above list sometimes it’s not all that good to be the prince!)

The prince is level 2 paladin, planning to end up as something like paladin 4/crusader the rest. He’ll have get-by ranks in the skill Use Magic Device, but he’ll have access to his retinue of a single-classed cleric, rogue, wizard, and druid… and a kingdom at his disposal. The prince also has time—there’s currently no pressure to wed and reproduce (well, no more than on any prince, anyway), so now’s the time to worry this out.

Additional binds

  • The term offspring is deliberately broad. Ideally, the offspring should be the product of both parents somehow. Outright duplication of either parent is permitted if A) the resultant creature can be raised from infancy (likely having no class levels), and B) the resultant creature remains capable of individual advancement by level or Hit Dice (e.g. it needn’t take the classes its father took, it needn’t have the same Hit Dice its mother had). (Hence the 8th-level Sor/Wiz spell simulacrum [illus] (PH 279-80) as printed is ill-suited for this purpose.)
  • For breeding purposes changing shape is suspect; if form integrity must be violated to perpetuate the aristocracy, a method of proving the resultant offspring’s parentage should also be provided.
  • The campaign uses traditional fantasy monarchies because that’s the trope and what’s expected. Adoption, while an interesting prospect and historically accurate in the real world, is certainly a solution, but has issues that need addressing. For example, a way to assuage the kingdom’s fear that they’ll live ever after under beholder domination were the prince and his beholder bride to adopt a baby beholder and declare it the prince’s virtual firstborn.
  • While the prince is progressive enough to consider breeding outside his species, the kingdom hasn’t embraced polygyny. However, the prince would like the widest variety of options. Finding a way for him to breed with a specific handful of the above creatures (or vice versa) rather than a general way to breed with all of them is interesting, but general answers are preferred.

Using any official sources except epic spells or an effect like the 9th-level Sor/Wiz spell wish [univ] (PH 302-3) et al. and by following the above binds, how can the prince secure his kingdom’s future by producing offspring with a creature with whom the prince can’t normally breed?

Please, for sanity and propriety, avoid anatomical details and play-by-play commentaries. Cast spell A, use magic item B, and dim the lights is more than sufficient, and if offspring can result with Cast spell A, use magic item B and no light dimming, even better. Advise the prince as to how to spawn an heir with the creature he married the language of class features, feats, magic items, spells, and other family-friendly, suitably vague methods.

(Updated substantially May 11, 2015; be kind to answers with earlier dates.)