While she says she will never do a threesome again she has developed a new outlet. She like to watch videos of guys with big dicks. When she has a few cocktails she has me use a dildo on her while she watches these guys perform. Recently she saw a video on here, a short video of a guy with a fat cock fucking a girl from behind. A pov type video of just his torso and her ass as she is offering her pussy from behind. She loves doggy but I have never been able to make her come from behind unless I reach around to manipulate her clit. With a big dildo she comes when I do her from behind with no other stimulation. When she watches a video of a lady getting from behind and I do the dildo she comes very quickly. During our only MMF the guy made her come while he did her from the back. Does it have to do with size? I have gone at it with her from the back 100s of time and she has never climaxed. Everytime I do it with the dildo she easily comes. She always says it was this or that why she didn't come but never says it my size. I know it must be and it makes me sad since she like that position better than any other and she always seems frustrated when she has to get on top to come. Any advice?