Lydia,
I know where you are coming from. When I was renting out my guest bedroom on AirBnb/, I usually felt more comfortable with females but could only specify that in my listing then screen guests as they inquired.
I am having a similar issue now that I am renting an entire condo on AirBnb. Often times, I get inquiries from individuals who have a good review or two, and use instant book to use my place. Little do I know, there are 5 (paying) guests who are under the age of 21 (the legal drinking age) and then I find beer bottles in my unit the next day. I don't want to turn off instant book because it attracts a lot of last minute stays but I there is no way for me to pre screen guests based on their age. I do wish AirBnb would give the hosts the age of the guest booking or I wish there could be a way to alter instant book requests.