I think YABU. You are thinking about it as if it's only you and would 'just' mean one or two extra people. But it radically changes the feel and the practicalities and costs of an event when applied to all guests.
I'm trying to arrange a wedding party right now. I'm at the age that almost everybody I know has kids and partners. If I include everybody's 'extra people', then for every person I am actually close to and would actually want to be there, I have to multiply numbers by an average of four. To have my best friend there is an extra four people! Making that choice means I cannot include my cousin at all- who I would like to include, but only see him infrequently... and If I included HIS extra people, would mean another four again!
That makes it totally unaffordable and also impossible to have in the intimate space I want. I'm also trying to hire a boat for a boat ride in addition to the party. The threshold In between the beautiful boat I can afford and the bigger one, which is neither as nice or as affordable is whether I say yes to a few people who want to bring their 'extras'.
It also transforms the feel of the event.
With adult only, (or even better, no plus ones!) you can look out as you marry to see every face somebody you are truly fond of and is truly fond of you.... with extras, you look out and see only 1 in 3 faces that you know and really matters to you. (And those are probably distracted looking after their kid!)
It goes from spending the reception really connecting with the people you love, to having to 'do the rounds' and check everybody is ok, has what they need, and feels acknowledged.
Unless your a particularly family-orientated person who gets joy from being around kids, it basically makes the whole event about pleasing other people, rather than having an experience you can afford and enjoy. And while it would be a natural choice to include kids and plus ones who you truly love and know, while excluding those in the 'outer circle' people would be offended by this as well.... which is why people create 'hard line' adult only, in the hope that nobody feels personally excluded.
Making the sacrifice of including all these extras is especially crazy when you know many of the 'extras' don't actually want to go and won't enjoy it! The plus ones often feel 'oh god another wedding? Really? Who is it this time?' And kids? Oh yes, they love getting squeezed into fancy dresses and hanging around all day, told to shush during ceremonies etc.....
Basically, including plus ones and kids, the bridal couple are sacrificing their event and their finances for your feeling that you have not been inconvenienced or discriminated against....
It's not about discrimination. It's about the couple trying to invite people they have a genuine relationship with, without ending up with an unaffordable jamboree that has lost all personal meaning.