Guest comics are acceptable in gag-a-day comics where each page isn't related to the pages around it, but BL comics tend to be long-form (have on-going stories, each page is part of something greater), and frequent guest comics are a bad idea for those. Even if they're done in addition to story pages instead of as part of the story, they disrupt the flow of the story for the readers.
Approaching strangers to ask them to do a page for you is also generally only acceptable if the comic is a really good exposure opportunity for them. If your comic doesn't have a large audience, then it's best to build up your audience with your own work for now. You can make an open call at any readership level, though (i.e. have a "submit a guest comic" page on your site with information). But again, make sure that guest comics are actually a good fit for your comic.