Yeah the sitting technique helps.
The bowel is not straight. It bends up - think where the colon goes - right up the inside of your body. I have fisted guys to my elbow, others with just a hand and there's no obvious place for the hand to go. We're all made differently and some guys never seem to quite have a sufficiently straight canal to allow deep anything. Shit coming the other way has lots of help - natural lube, softness (hopefully) and muscle contractions (bowel movements).
The distance between the external sphincter and second sphincter varies. The internal one is not under voluntary control. As someone pointed out, it's function is to open with the flow, as it were, so sometimes causes a problem when a dick/dildo goes in.
i do get this problem with longer dicks too - it's not the internal sphincter, but the angle of attack just hits a dead-end/back of the lower colon. DON'T force this issue. The colon is quite delicate, lined with a unique smooth muscle. It tears and takes an ge to repair. tears are also potentially very serious health issues if fecal matter passes into the body cavity. DONT FORCE.
It's just the angle. I also find the doggy position helps more, straighting the bowel. And take control so you do the pushing - the "active" bottom.
Poppers may not be everyone's favourite but their role, dilating blood vessels and relaxing everything, do help accommodate bigger dicks, in my experience, especially getting in first time.
Breathing tricks also help a lot, being relaxed, calm etc. Gentle pushing out will help, but yes that could become a bowel movement. Well, shit happens, so be prepared to re-wash.
Lube? More lube, and even more lube. Douching washes out the natural mucous we make to aid the other function of the colon, passing the waste out. So if you want to slide in the other way, better replace the mucous with lots of water based lube for condom-wrappped sex, or a non irritating oil-based if it's condomless (leaving aside health issues). Purposes made lube for anal really do make a difference.
Also do not use condoms designed for straight sex that contain anti spermicides like Nonoxynol-9/. They irritate the ass lining and make sex less safe. Use ones with none or designed for anal sex - and they can be standard thickness, it has no effect on the safety of the sex.
Good luck. Resistance is futile the Clingons once said, well what they would know about butt fucking?! [SIZE=-1][/SIZE]