A missionary dater is one of a religious faith, commonly Christianity, dating a person with differing beliefs (or commonly no beliefs at all), for the purpose of changing that person's beliefs or religion.
Carrie: There are far too many guys out there that don't have my same belief system so I shun them. But aren't I called to spread the good news..? *sigh* Life's busy and time too precious to waste it dating a nonbeliever...
Casey: "Heckkk no. I ain't no missionary dater!"
A condition which occurs mainly in men, and results in red, slightly sensitive elbows, caused by prolonged friction with a couch, while in the missionary position.
"Hey Mark, did you see Steve? He's got some nasty Missionary's Elbow."
"Yeah man, he must have had a good time with Claire last night. He's got a hickey too."