Pregnant Man Thomas Beatie is getting really serious with his new girlfriend, so much so that RadarOnline.com has exclusively learned he just bought her a large diamond ring, they're living together, and he's even having his three children call her "mom!"

Thomas, 38, and his wife Nancy recently separated because he began a relationship with Amber Nicholas, 42 -- the director at a Tutor Time daycare center in Phoenix where his children formerly attended -- and Nancy is furious over it.