I agree, if it is about trust, there might be things which need to be discussed, but there could be other things in play.I suppose there are other factors like they don't like the other person etc - but if it was a case of "No, you're not riding with Sally because I don't trust you" then there are serious issues that need to be addressed.
I think the most obvious alternative might be along the lines of "What would the neighbours say?", which may raise another set of questions.
Yup, pretty much exactly that.It doesn't have to be about trust, your wife can trust implicitly that you would never cheat on her and still feel uncomfortable that you have another woman you share your free time and favourite hobby with, another woman who gets part of your life she can't relate to and understands part of you she can't. Some would be bothered, some wouldn't, but with all the compromise in a relationship I don't think its the biggest issue to worry about.
I don't see a problem with a platonic relationship though, so long as its not upsetting any other halves.