Rosie O'Donnell isn't as pissed as Lance Bass about gay men not being able to immediately donate blood after a tragedy -- she says the nation's gun crisis is the bigger issue right now. 

We got Rosie Tuesday leaving LAX where she had zero enthusiasm for our photog's effort to follow up on Lance calling out the FDA for its 12-month blood donation ban on sexually active gay men.

She's clear on the history of the blood ban, and while she has compassion for Lance ... Rosie's mind was more focused on a guy being able to shoot 500-plus people with weapons that are far too easy to get.