This particular comic isn't so much funny as it is heart-wrenching. No one likes to get shafted, especially in Scheme.
Proposal jokes are a favorite of mine (see my December 1993 "special" comic) simply because they're so versatile. There's almost an endless supply of twists one can put them. A proposal of marriage is such a high-stress endeavor to start with, that it doesn't take much to push it over the line into absurdity -- as I've done with this drawing.
Incidentally, my proposal to my wife was nothing like this. In fact, I made sure of it. So great was my fear of rejection that I actually asked my wife several days beforehand if she would say yes if I asked her to marry me. If there was even a small risk of her saying no, I simply wouldn't propose. Herein lies a lesson to be learned, for any aspiring grooms-to-be... if you don't know she'll say yes, don't ask. Such uncertainty is a sure sign that you need to get to know each other better before considering such a long-term commitment.