DS is 12 (turned 12 in feb) and just finished 6th grade. some of the kids he knows are bf/gf, and a couple have kissed...but we are talking pecks, not making out. and the bf/gf stuff seems to be just like when I was a kid oh so many years ago...passing notes, texting (well, no texting way back when, but I see that as similar to notes/phone calls). they didn't really go anywhere. in fact, it was a huge deal when one of ds's friends actually DANCED with his gf at the end of school dance. all very innocent. and still I'm happy that so far ds has zero interest in even that. plenty of time in the future!
as to the op's original question, I wouldn't have a problem. but I guess I envision it using the kids I know/parents I know involved. we have had parties in the past where girls and boys were together, and parents were over and had some wine. they were good times. have been to friends homes in similar situations. parents are chatting, kids are usually playing manhunt and capture the flag, etc. tons of fun with flashlights. they run in and out of the finished basement, where they usually are, while the parents are upstairs.
I'm sure other places and other kids are different.
as to the op's original question, I wouldn't have a problem. but I guess I envision it using the kids I know/parents I know involved. we have had parties in the past where girls and boys were together, and parents were over and had some wine. they were good times. have been to friends homes in similar situations. parents are chatting, kids are usually playing manhunt and capture the flag, etc. tons of fun with flashlights. they run in and out of the finished basement, where they usually are, while the parents are upstairs.
I'm sure other places and other kids are different.
What's the rush to engage in adult behaviors? There are so many other things that kids that age should be doing. Rules and limits are going to be much harder to enforce when they enter the teen years. That's when the real "fun" part of parenting begins. 
and judge because it was nothing more that holding hands when walking from the bus to class, her 
I say that because socializing is a big part of learning. Just because you put boys and girls together doesn't mean that they are all going to re-enact Eyes Wide Shut.
I think with every party whether it is all the same gender or not- there should be supervision. OF course some parties more than others.