Well it depends.

For example my ex-girlfriend was less than exciting in bed. For a while it was great because we had the whole love thing going on so sex was focused more on that, and as a result it was always great because it was nice to feel close to someone I loved and to truly make love to someone. But after a while the whole love things reaches a plateau of sorts, and just going at it missionary can be less than exciting. I mean its great when its new love and you can whisper sweet things to eachother and all that jazz, but after a while it becomes about phsyical pleasure.

It just seems she didn't have much desire to really do anything, and the same two positions [me on top or her on top] just going through the motions til we came got less than exciting.

So if its about someone who just isn't into it, or doesn't really care to get into it that could end up being something any guy [or girl] is not pleased with. However if its just lack of experience, but the will is there, it's all good.

Me and my current girlfriend get it on both ways depending on mood. Maybe we have had a lot going on, and didn't get to see eachother too much, so we spend the day together and work our way to bed feeling all lovey dovey and just do something simple but are left siling because it was what we wanted. Other times we rip eachothers clothes off and I'm spending time standing beside the bed holding her legs against my shoulders, before flipping her onto her stomach and going for a new position.

Simply put, if its just vanilla sex for a lack of willingness or desire, its' not gonna be fun for too long. If it's just a lack of experience, time will get you there and you'll have fun doing it.