While I realize you are speaking about your experience I would gather that for most people they don't confine time spent with boyfriends as weekends only. Even during the school year (though the time to go home would likely be earlier than for weekends if it was a school night).
Isn’t that what most ppl have done on the thread...express their opinions?? That is just my opinion. Most ppl have been downright dismissive of the OPs feelings on the subject & told her to lighten up b/c that’s their opinion & experience & implied that the way it’s done in their family is the “right way”. That’s great if that’s how some families are. That has not been my experience & so not what I prefer. Just my experience & opinion.
To me it wasn’t whether it was ok or not. It’s that I thought it was a stranger request once the OP disclosed more details of the party. Although I understood her being annoyed with the request at all, I thought it was a little more reasonable of a request when I thought it was a casual party on a weekend.
I can understand the casual aspect but I think most people still consider it a more casual affair in the end given what the event was about (1-yr old bday). The OP themselves said "It's "formal" to the extent that, until two days ago, I had assumed I would at least know everyone in attendance." It sounds like the OP has made several choices out of necessity due to the number of people (prior to the bf issue) to use their China dishes that they would eat casually with too, tapas because they were convenient, ready to eat or heat and serve style food, etc While I think in the OP's mind it's more of an adult feel rather than a more child feel that people tend to envision with a young child's b-day party I somehow doubt that the 16 year old bf would ruin the overall adult feel.
This! I've never been to a child's birthday party (and believe me, I've been to dozens) that was a formal sit-down dinner. Even if someone said "don't eat before you come over because we'll be serving dinner..." I'd still expect pizza or burgers/chicken on the grill or something "casual". I can totally understand how SIL isn't batting an eye at asking to bring the boyfriend along...they're probably not expecting their dinner to be served on china while sitting at the dining room table (but "yay" for using the china on a day other than Christmas or Easter!)
The older I get, the more I use my fine china. My grandmother passed it down to me, and I was able to (almost) complete the set using replacements.com
I'm sure if not this weekend, then we'll catch up with them soon.
, showed off his sweet new dance moves, had a blast playing with his new toys, and discovered the wonders of these things called balloons. 
No, he’s driving back down today. It sounds like he had a great time exploring old Montreal, beer garden, ice skating, was going to a club last night.
LIES!!!
Perhaps. Of course, it’s only natural that I would know what it is typical in my circle. But, I also don’t think it’s outside the realm of possibility that things are just different in my area. I’ve noticed that with other topics that have been discussed on these boards.
