would there be any medical concern with me sleeping with my dick inside my girlfriend’s ass?

for context, we're both trans girls so there's 0 risk of pregnancy, we both got tested plenty times and don't have relations with other partners so we decided we're pretty clear to do stuff without protection. she's suggested we sleep together with my dick inside her. i remember reading long ago about how the vagina has a ph difference and could harm the penis if contact is kept for too long, does the same thing apply to the anus? could i safely spend 8 or so hours with my dick inside?