Plus one Rant

@FutureMrsM, I don't think you can spell it out for people that way on the invites, it will come across as rude and a bit accusatory. Here are a few ways we kept random plus-ones off the guest list:

1. Word of mouth and setting expectations. From the day we got engaged, we told anyone who asked that we envisioned a small, intimate wedding. We didn't want to look at photos twenty years from now and wonder who the heck people were, and I didn't want my stage fright to act up from a room full of randoms.
2. We put the invitee's name only on the envelope. The RSVP card said "__ seats have been reserved in your honor."

3. We had guests email or call to RSVP (easier with a smaller wedding), so any attempts to add a guest could be immediately and gently turned down.
4. If a guest asked about a plus-one (such as a newish relationship where I hadn't met the person yet), we told the guest we B-listed that person. "We can't accommodate additional guests right now, but once the RSVPs are all in I might be able to swing it. Can I let you know?"

5. Have a script and stick to it. DH and I would actually practice saying, "We have a small wedding planned, both because of budget and venue restrictions and because we envisioned a more intimate gathering." Then any time we got cornered about a plus-one, we just rattled off the script then changed the subject. If you let the person continue the discussion, they'll try to argue with you - including one (to-be-unnamed) single GM who wanted to bring a random date, to a wedding where she wouldn't know anyone, and even offered to pay for her to attend. Holy awkward.

To add on the Shannon S. suggestions,

We had our calligrapher literally write out each invitees name on the RSVP card. We haven't any issues so far. Well except the chic that wanted to invite her brother in replace of her boyfriend, but my FH shut that down.

Yeah, your in laws were rude to corner you like that.

A 2-year rule on a relationship does seem arbitrary though. For instance, after 10 months of dating, I knew I would probably marry my guy. That means it was a pretty serious relationship and it was much less than 2 years.

I really don't get why people get so upset at not being able to bring a date. I've loved attending friends and family wedding solo, because I didn't have to continuously make sure my date is having a good time. My FH and I went with the guidelines of married, engaged or living together (at the time the invitations were being sent). One friend has emailed saying that she was offended she doesn't have a plus one. We've never met her new bf of 6 months, they don't live together and I've only heard her talk about him twice in the past few months (we live in different states so we don't talk too often). And honestly do not have the space for another person. I emailed back and let her know how much it means to me to have her in attendance, and that I couldn't imagine the day without her there. I then added that the need to limit the guest list is a sucky aspect to wedding planning and that we too are frustrated that we are unable to invite everyone we love. That helped to put things in perspective. While I want all my guests to be comfortable, I not going to not invite another close friend so that another friend can have a date. That being said, if we get enough "declines with regrets", I'll happily extend the +1 because I know it will add to this friend's comfort level at the wedding.