Thanks for your comments, guys. I feel like I want to clarify some things, just in case i wasn't clear enough before. First of all, my sister only told me about her and her husband's reaction because I raised the issue, having been told by my mom. She didn't make any requests of me to tone it down. It was my mom that asked that I take my time with showing physical intimacy with Ben when first introducing him to family. I'd say "bigotry" is a rather strong word for this situation.
Basically, most of my family is very conservative. Not religiously conservative for the most part, just rather traditional. They have very little experience with homosexuality. My mom, when she suggested I slow it down, even said that "it shouldn't be this way," referring to the double standard (i.e., if it had been me with a girl no one would have had a problem with me getting "touchy feely," especially if it was something so simple as an arm around the shoulder or a hand on the leg.
My sister is an amazing person, and she took my coming out very well. My bro-in-law is kind of a stereotypically "macho" kind of guy, so I understand if he has homophobia to deal with. My sister only told him about Ben and me a couple weeks ago. He probably needs some time to readjust to this new information, so I'm willing to give that to him. He sure has selflessly helped me out many many times.
I guess the way I feel is this: I was disappointed by their reaction but I don't blame them for it. If anything I overestimated their ability to adjust, but everyone's different. I'll give them time to get to know Ben better, and will probably also sit them down and have a talk at some point (definitely avoiding words like 'bigotry' and 'homophobia' though, haha). I'm not sure that the best course of action would be to act with Ben exactly as I would act if he were a girl, at least not right away, because we just don't live in that ideal world (yet). As for my nephew and niece, I guess I'll defer to my sister as far as the timing of giving them a talk.
As for how they'll get "used to it"? Well, I guess after they've had a while to get to know Ben I might gradually start ramping up the minor PDA and see how it goes over.
Last order of business: check out http://broandcollege.blogspot.com/, a new blog by a guy I've been chatting with online.
Take care everyone!