I have a question concerning ukes in mangas.

I have been lurking this site for quite some time and i have seen people being bothered by a few things when it comes to yaoi mangas. Here's what it really bothers me:

Why is it that in the majority of mangas is it somehow a norm and ok to show the uke having sex with other people but not the seme?

I have read lots of mangas and i have encountered this i don't know how many times and it really pisses me off. When it comes to sex the manga artists don't really have a problem with depicting the uke being sexually active, having sex or enjoying sex with other people , but when it comes to the seme we are always told that he was a playboy or had sexual experience but very few artists actually show the seme having sex with other people beside the uke.

I wonder why is that? Does anyone know because i have dropped so many mangas because of this stupid attitude. Either show both of them having sex with other people or no one at all. This double standard is making me crazy.

I don't mind if the seme or the uke had previous experience or relationships but when i am introduced to them in the manga it ruins my mood so much when i have to see one of them, predominately , having and enjoying sex with other people. How am i supposed to believe that then they have feelings for the partner? For me it cheapens the relationship so much and i can't root for the couple any more.

Does anyone knows why the manga artist do that? I'm really curious about it...