The reasons why couples seek single females are various and usually have nothing to do wtih the male being insecure about having another male around.
We swing with singles as well as couples (that includes single males as well as females and more often than not single males). Personally, we like 3somes better than 4somes because it allows for all of us to be involved moreso than a 4some would where we end up off in two pairs. As for a single female... I'm bi and I'd love to have a FMF threesome with my hubby.
There are couples who do swing solo (without their partners) and there is nothing wrong with that if that is what you are comfortable with doing. However, pushing someone else do so when they aren't into that isn't cool.
Whether you are a couple, single male or single female read an ad before you respond and make sure you are what they are looking for and they are what you are looking for. If someone responds to your ad that shouldn't be, let them know that that's not what you are interested in and that you put that in your ad (maybe if they hear it enough they'll stop chasing the wrong people).
Julie
Http://www.swingersboard.com