Now my turn to be brutally honest:
1) This is not about what other people want.
I’ve always felt that other people place way too much importance on what /they/ think is appropriate for someone else’s wedding. It’s not about them, it’s about what you and your fiance want. It’s not their decision. You’re an adult and you aren’t going to make your wedding–or your life, for that matter–into what they want it to be. This is your chance to set boundaries and parameters for your relationship with his family. If you don’t start setting them now, it will just be more difficult later on, for example, when you have children.
2) You are doing /nothing wrong/ with what you have planned.
That said, however, I think you are giving his family too much control over your special day–you are so afraid of them–and also you aren’t giving them a fair chance. I would be a little upset if I were them at not being invited at all to the ceremony. I understand you don’t want them to come because it’s secular, but I think they should at least be given the option. If they choose to be offended that it’s secular, then they don’t have to go. If being offended by that is more important than seeing their son get married, well, at least you tried.
3) Yeah, they’ll probably be offended. But, on the other hand, who cares?
My husband and I had a ceremony in Vegas with only my parents, his parents, and my sister. We had a glorious reception back home three months later and it was wonderful! Everyone had such a nice time and there was nothing to be offended about.
4) They will deal and get over it.
My parents who are strict Catholics. They asked early on if it would be Catholic. I said no because my husband and I are not religious, and having a religious wedding is not appropriate for either of us. They never said anything again, and they had a great time at the wedding, anyway, my mom even gave us a beautiful toast at dinner that really surprised me. In the end, their own desires for what my wedding should be did not matter because they were just happy for us. I’m sure the same will happen with his family once they see that they will not control what you are doing.
5) Wording the invitations is easy.
Whether or not your ceremony is right before or after your reception, it is very easy to word the invitation. On our invitation, we just requested the pleasure of their company at our wedding reception. I can give you the exact text, if you want, in a private message.