A) As others have said, sluttiness is a matter of opinion. You may not wish to do it, but if she is enjoying it, why the hell should anyone else care?
B) I've had time for numerous long term relationships (I say long term is 1 year plus at my age -23), and have also had the time for a healthy number of one night stands and casual sex.
C) Being in a long term relationship doesn't mean you are better at having sex, or giving head. Perhaps, if you have a relationship with good communication, and you are patient, with a healthy side dose of being adventurous and experimental, then there may be the possible that with that person, you and they may have great sex. I don't believe there is any such thing as being good or bad at sex...I believe in sexual compatibility. Some people you will never be on the same page as, and it will be rubbish. Some, it will be liquid, fluid = amazing. If you /they have an amazing time in bed in that long term relationship, the things you know and they like won't be the same with the next person. However, if you have continuous one night stands, you will indeed have a wider range of skills to draw upon, that work for a larger number of people. However, you wont of had the opportunity (well, unlikely to of) to experiment and therefore will have a limited experience.