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In short, any Venturing activities that have male and female participants (specifically those under 21) should have co-ed adult (over 21) leadership.
Now, that being said, my crew had similar scenario when it started up. When we had less than eight active Venturers, we would frequently have only only four to five attending overnight activities. The ladies in the crew had fathers who were advisors, and the other guys were either brothers or grew up with the brothers through Boy Scouting. For the first two years, our female advisor was very transient due to her job hours, so we made the best of situation. We still attended the activities our Venturers planned, and we may have broken some YPT in process. However, we had a very open and comfortable culture in our crew, and it worked for the benefit our youth! One of those ladies went on earn her Silver Award and work at Philmont. Bare in mind all this was also 5+ years ago.
So, I'm by no means advocating ignoring YPT, but you should also look at the dynamics in your crew. Take that how you will.