things in anime you get sick of seeing

[Rosencrantz]

Also - while we're on the topic of relationships in anime, I have to admit, one thing I always find odd is that no matter how much fanservice you get, characters never seem to actually have sex.

Sex is something you have to commit to plot wise, your typical fanservice show would fall apart in seconds as the premise basically relies on none of the girls having quite won out over the others. For other shows you have to consider what adding it will achieve. I've only seen Clannad from the list you gave and in that it's a natural progression of their relationship.

 

[ilmaestro]

I wasn't paying attention and assumed ilmaestro was saying that Index is a show which tries to include more sex at first :/

I edited in Rosencrantz' name afterwards, so you might not have been going entirely crazy. ^^;

 

[Smeelia]

I think one of the reasons that sex is rarely bothered with in relationship stories is that it doesn't usually add much. Stories interested in dealing with fears about intimacy tend to just use kissing and that generally works fine (with the added benefit of not messing with the rating of the show). There may be practical health considerations with sex and an examination of attitudes, techniques and so on could also be interesting but for the topics that romances and such are generally concerned with there's not a huge difference between sex and holding hands (aside from those aforementioned ratings issues).

As for sex relating to fanservice, I guess the reason sex is avoided in fanservice shows is that they and actual porn are kind of different things. I guess you could have implied sex in fanservice shows but I don't really see that there's much benefit (but then I also doubt the need for an audience stand-in protagonist so maybe that'd be more a matter of taste).

Basically, sex is either likely to be the central subject of the show (rather than relationships, emotions or whatever) or else it generally becomes unnecessary and loses out to options that have added benefits.