Damn it. I want to see their relationship take a more physical turn. We've had enough drama, time for some sexy time!
To be honest, and when considering the manga thus far, I do not think what you're looking for is going to happen. I do believe that at some point Mei and Yuzu will have sex in the story, but when it does happen it will not be heavily explicit. My guess is that the moment will be mostly implied with some before and possibly after scenes. If you're just reading for sex then there are plenty of yuri hentai out there and I'm sure some fan out there has drawn Yuzu and Mei going at it.
One thing I really like about Citrus is the realistic set-up of the sexual content. It is easy to be facetious with sexual comments and actions before a relationship starts, but once love is discovered and the serious nature of the sexual actions are revealed in a relationship, often times a couple, especially teens, will take a step back just like Mei and Yuzu have. It's refreshing that they are not rushing into it and are taking time to learn more about each other first. Let's keep in mind that the targeted audiences for Citrus are lesbian or bisexual-female teens.
Underneath all of the repetitive drama, sexual jokes, and absurdities that this manga has, there is a serious romance where there are characters with heartfelt issues. Yuzu even gets scared thinking of how society will look at and possibly condemn not only her sexuality, but her relationship with Mei, her step-sister. I also have a growing thought that Yuzu and Mei's mother will grow suspicious of the closeness and start becoming more involved in the story.
I'd also like to see if there will be any development between Harumin and Matsuri, and I'm looking forward to having Harumin's sister present at the sleep over.