NellieMoser - take no notice of the disdainful response you received -
In my experience, choral come-and-sing days are brilliant - go for the Hayden Nelson Mass if you are familiar with choral singing, or want to try it for the first time (just look at the music if it is provided, and open and shut your mouth as if you know what's going on, that's what our community choir leader advises).
(At the last event I partook in, an old hand told me that quite honestly, a lot of people in the Choral Society were often a bit all over the place, it wasn't just the newcomers!)
You might have been unimpressed by a previous performance, but if you are familiar with the format or are adventurous, trying out a new experience, then go along, snuck in and try it out - as you say, come-and-sing days can be great fun. I've sung along in the last two years, in several events (Brahms, Mendlessohn, Elgar, Handel) led by our local Choral Society choirmaster who is very welcoming to newcomers (although often ascerbic with his comments, as so many choirmasters are! - take no notice, I think they all aspire to be prima donnas!)
And if we are not sure about what is going on we just keep quiet - best bet is to stand next to someone who really knows the piece!).
You refer to "a disdainful response from someone about it being an "amateur performance". You say "I was too gobsmacked to reply. In Britain we have a fantastic tradition of choral societies made up of non professionals and many of them are very good indeed."
Forget the sender of the disdainful reply. There is a lot of snobbery around classical music but fortunately it is dying a death (slowly). Community choirs are moving in, infiltrating and allowing for more than one style of choir singing to blossom? See the Proms - all the classics and all the modern composers - alongside musicals, Gilbert and Sullivan, film music. What's not to like? Excellence in all musical fields, genres, whatever, is wonderful. 
PS - I had to Google for the spelling of Mendlessohn. Hope that's right.
PPS - about female tenors -
A few years ago I came back to choral singing (I was a soprano when young, good enough to do serious school solos) after a gap of 30 years, and sang as a wobbly alto at a day choral-sing-along. There was a woman there who sang tenor. Wow, I thought, I feel comfortable singing down there.
So a year later I went along to a come-all-ye to sing Haydn's "Creation" (which I had sung many years ago). "I would like to sing tenor" I said to the choir master. He laughed in scorn. I never went back.
A few years later I tentatively joined my current community choir and said "Can I sing tenor" - no problem, I was welcomed aboard. We have a number of female tenors. Tenors are always in short supply and although we do not (yet) have the strength and power of male tenors we are welcome. So any female tenors, go for it.
Suggestions please for large pot.
