Oi vey I can only imagine the agony teenage girls must suffer that wind up trying this with their no-doubt insistent teen BFs... If you thought a hyper-active teenage boy (i.e. idiot) rabbit jack-hammering away in your vagina was tough going, I can only imagine how much worse it will feel in your anus without proper preparation and a gentle ramp-up!
That's the one problem with the porn industry and the rise of gonzo / extreme porn: there's a lot of ass-fucking, ass-to-mouth, Double penetrations (DP), double-ass penetrations (DAP), etc. but they don't show you the prep work and essential hygiene the adult starlets undergo to prepare themselves for the shoot. Then you have idiot kids trying to copy what they see and are in for a shock (and a painful, messy and often embarrassing one at that).
Key things teens/people need to know before they muck around with anal sex:
i. Their bowel movement schedule (you don't want something coming down the pipeline when you're getting down to business)
ii. Learning how to perform an enema (unless you like particulate coming out at awkward moments)
iii. Investing in dark condoms (residue shows up less on black/dark condoms)
iv. Front to back is essential for her vaginal health (never go back from anus to vagina unless she somehow has really cleaned herself out or is willing to risk the bacterial infection)
v. Never go ass-to-mouth for the same reasons above, plus just gross (but hey, if that's your kink, go for it...)
vi. Start with your smallest finger and let the sphincter become accustomed to the insertion, then dilate it further by gradually increasing in finger size/quantity
vii. A modest-sized toy or two (butt plug, smooth dildo) as an additional dilation step prior to penile insertion will go a long way too
viii. "Just the tip" means just the tip - a good starting place until she gets used to it
ix. Then proceed SLOWLY with her reassurance and only increase in tempo when she relaxes and feels ready
x. Lastly, massage that clit! The more relaxed she is and the more stimulation she gets, the more likely she is to orgasm while doing anal and be interested in trying it again some day
Now go to it kids, and good luck!