Replying to Hard Decision

alright as you guys know ive had many questions about me and my girlfriend..im still with the same girl and she wants to make our relationship more interesting..weve been seeing each other on and off for about 2 years..so today i goover to her house and she tells me she has a serious question..i figured it would be about how she thinks we should only be friends..i am proved totally wrong..we sit down on the couch and she proceeds to say that she loves having sex now but she thinks we need to "spice" it up..she said that i can fuck her whenever and wherever i want from now on and get to have a 3some if i agree to one thing. now all of this sounds great to me so im eager to say yes, this doesnt sound like something that happens to you very often. well her question is that if i agree to her, she gets to fuck me in the ass. i dont know why she wants to or what she would even do it with. im assuming her dildo but im not really for it and i dont know how long it would last. i asked and she said until she gets off, which could be 10 minutes or 2 hours. so im in quite a predicament. i think i love her and i would do ALMOST anything for her. so heres the question ns...should i please her and let her fuck me in the ass ONCE, or say no, which wouldnt kill the relationship but i wouldnt get sex whenever i want. thanks to all of you who respond seriously