Normally, you can have a NOT tag, to specify that you don't want something. For example, an exhentai search for
male:shotacon language:english
will net everything with shota, gay and straight. If you just want gay shota, you would search
male:shotacon male:yaoi language:english
and you would get just gay stuff. Exhentai has the "-" modifier for their NOT command, so what you want, is
male:shotacon -male:yaoi language:english
Now, this will take away all bisexual content, as well as taking away tankoubons that have 6 straight stories, and 1 gay or bi story. Just something to keep in mind.
On gelbooru, you can just search for
straight_shota
or you can search for the type of woman you're looking for, and then type
shota
after it. So if you want to find dark skinned BBWs with their shotas, you would search for
plump straight_shota dark_skin
Keep in mind, however, that this will return results of dark skinned shotas, as well. Another thing to keep in mind. Gelbooru often makes odd choices for their tag names, and their tags narrow down, they don't expand out. So something like
curvy straight_shota dark_skin
will net similar, but distinct results, when compared to the search that we did prior. It's kind of a hassle to keep track of all these tags, but you'll get the hang of it eventually.