Can a male who's 5 feet 7 140 pounds pick up a 100 pound 5 feet 1 female? That's his gf ? Or does he need to get muscular or fatter?

If a man who's 5'7 has managed to attract a girlfriend in the first place he should feel proud, regardless of whether or not he can lift her up. If she chooses to be with him despite his shorter height it's likely that she's more interested in him for his personality than his looks or strength, which should be comforting to him knowing she's there for non-shallow reasons (including his ability to lift other humans off the ground). Also, women tend not to like when short guys get too muscly because it looks like they're overcompensating.

For sure..

Well there's only one way to find out. Do it. I'd say that's going to be difficult.

Well, when I was in the 9th grade I stood 5'6" and weighed 135 pounds...and could bench press 240 pounds. I'm now 60 years old and stand 5'5" and can bench press 320 pounds...so I don't think I'd have a problem picking up your girlfriend if you can't.

Possibly.