There is a serious effort underway to block OctoMom from taking a million dollar offer to do porn -- except the opposition is coming from a rival XXX company!

Fearing that Nadya Suleman "will become the subject of endless ridicule and scorn," the president of Pink Visual has offered Octo a "full year's worth of diapers" if she keeps her baby maker where it belongs ... fully covered and far away from any type of recording device.

In a letter sent to Octo, morally righteous Pink Visual warns, "There's a great deal of stigma attached to being a porn star" and that Octo needs to think "in the best interest of your children." For once.

So what's it gonna be? Do porn ... and get a million bucks, health insurance and benefits -- or keep it covered ... so you can keep your kids clean? Decisions, decisions.