Summarize:

Whining may cause your parents to treat you like a kid, so it is important to avoid whining. Act like a mature, young adult by asking your parents about having a coed sleepover in a calm voice.  If it looks like your parents might say no, don’t lose your cool. Continue to speak respectfully and let them finish speaking before you jump in and defend why you want to have a coed sleepover. If you want to host a coed sleepover, make sure that you have all the details they might want. Asking your parents to have a coed sleepover before working through the details may make them think that you’re not mature enough to have a coed sleepover.  Have a time and date available when you ask your parents to have a coed sleepover. Make a list of all of the people who you want to invite to the sleepover. Figure out where everyone will sleep. Your best bet is to tell your parents that the girls will sleep in one room and the boys will sleep in a different room. Don’t pressure your parents into making a decision right away. They may want to weigh the options of having boys and girls sleep over, so give them space and time to think. Don’t bug them or they might become irritated and just say no. Offer to let them talk to any other parents in the meantime to find out procedural issues Do all of your chores and make sure all of your homework is complete so your parents don’t even need to ask. Working in advance to show your maturity is a good idea so your parents don’t have to ask you to do anything.   Do all of your homework in advance so that you don't fall behind on schoolwork. Offer to help out your parents with things like cleaning, cooking, and running errands. If your parents still seem reluctant, then you might consider asking if you can have your girlfriends sleep over and just have the guys come and hang out. Your guy friends could stay until a certain time, such as 10pm, and then head home.  Keep in mind that this will mean that your guy friends will have to arrange to get rides home. Tell all of your guy friends in advance so they can make arrangements for rides.
Speak calmly and avoid whining. Be prepared with all of the details. Give them time to think and talk with other parents. Show how responsible you are. Offer to have the guys go home before you go to bed.