If you've ever done anal with a girl then your point about putting your penis up someones butt is moot and negated, as stated earlier an anus is an anus male or female they're exactly the same. Plus I assume you'd be wearing a condom (safe sex only ofcourse) so really since you'd be behind your person (we'll assume they're an effeminate guy with long hair) with the right lighting you could easily picture a girl there.
As for not being able to get it up? Well BoB theres a simple answer I'm surprised no one mentioned it earlier, VIAGRA solves this problem in fact you'd probably be able to earn a million £ (I'm english and want my payment in £'s) tip for going so long and being so hard
Taking it up the backside is something that I suppose is a matter of personal opinion. Done correctly it wouldn't hurt. (A lot of guys are secretly quite curious as to how it would feel and would accept a woman to use a strap on, strap ons are generally bigger than the real life deal and in this case guys, size will matter) To be honest I believe all you need to be able to do is relax, if you're tense about the idea then it's not going to happen easily and painlessly, perhaps poppers would help with this after all they're designed to help with this and to kill braincells.
Would I have sex with another guy for £10 million? Yes I would have sex with a guy for that so long as I don't have to perform oral for them. I'm fine doing oral for women but for men it's a whole different ball game, theres no closing your eyes or adjusting the light to pretend it's a girl you're riding or being ridden by.
So yeah call me a whore, a slut or whatever you will but end of the day, since the £10m payment is tax free because it's an illegal act and who announces they got £10million through an illegal act? (btw thats richer than Rupert Grint, actor who plays Ron Weasley in the harry potter movies who only has £7 million) On £10m with correctly investing the money and not going insane and spending it all you could live a life of luxury for the rest of your life and build upon your money so your kids and their kids have something to inherit. Plus like my mom always used to say: Don't knock something until you have tried it.