Any suggestions on how to answer " why dont you have a serious boyfriend?

I am a nursing student, studying for my Bachelors to get my RN. I am working at the hospital in my home town this summer as a certified nursing assistant. I continually get this question both at school, socially and at work. A girl who looks like you doesnt have a serious boyfriend? Im not sure how to answer this. Nursing school is pretty demanding and requires alot of studying for me. Also I am interested in a special guy who is also very focased on his education as well as his college sport. I am asian, small, athletic, a bit old fashioned morally, and have had people tell me im pretty, i dunno. I do know a couple of guys who like me and who would go out with me...if that was all that I wanted a guy by my side...but i know that I dont have special feelings for them and dont want to play with someones emotions. Ive had it done to me in the past.. So what can i tell people when they ask me why a girl like me (whatever that is ) doesnt have a serious boyfriend?

The perfect response - "Why would you like to know? Do you have the perfect guy for me?" That will either shut them up or get you a date!!! Don't worry about those people, finish your education, get a great job and the rest will fall into place. I have been in that exact situation and I know it's annoying but try not to let them bother you!

tell them to back off and mind their own business. Is there a time limit on getting a boyfriend? Tell them that you have more important things to think about like getting your qualifications and a career.

First of all, it is none of their business. It would be perfectly acceptable to tell them that.
If you don't want to be rude and nasty (as I probably would be LOL) tell them that you don't have time right now. You are committed to your work and schooling. There is absolutely nothing wrong with that.
You can also tell them that you are still waiting for that special man to come along.
When they say 'a girl like you....', correct them!! Tell them that we are all special people in our own way, and having a pretty face has nothing to do with having a boyfriend. You want somebody to like you for who you are, not what you look like. These people are all being rude!! You don't have to put up with it. I admire you for setting goals and not letting anything get in your way. You don't need a man in your life to complete you or make you happy - or to support you. Keep doing what you're doing!!
Good luck!