sadly a lot of yuri writers don't have a lot of imagination when it comes to making sex scenes that are different haha. always the same thing too! once they go down on the other it's always like, "no, it's dirty!" lol
The "no, it's dirty"/"no!" and "stop" during sex thing is something inherent in Japanese culture, so it sorta makes sense it's a thing in Japanese manga. Not the best source (or most official one), but if you've ever watched JAVs and the like you'll see it too (girls going いや or だめ！). This happens in yuri, het, and yaoi manga where there's sex, and it's there for a reason. Cleanliness is really important in the culture as well, and girls take showers before having sex (and after it). From http://www.quora.com/Japanese-language/Why-does-my-Japanese-girlfriend-say-dame-dame-when-shes-experiencing-sexual-pleasure:
In Japanese society, it has been widely believed that to be innocent is one of the appropriate [characteristics] for young ladies...it means "Yes", and the long version is "I am (trying to look like as if I am) too innocent to accept this kind of shameful situation!"
As for the lack of imagination, it's more of a lack of things to do. You could argue that toys could be used, but not all couples like toys, and unless they're into BDSM there's not much more to do. It's not like with a straight couple where you've got two sets of different goodies to play with; you've got two sets of the same in yuri. Not that it makes it worse or less interesting, but there's just less combinations for variation numerically, so of course there's less variation in general. Sex is really pretty much the same thing at its core anyway. Sex won't be the same every time, but it'll be pretty damn similar in terms of procedure, so expecting it to be completely different every time is unrealistic, unfair, and pretty unfounded.