Even when I was a JW I never made 'sisters' sit in the back of my car. I viewed it that to do so would either mean to the sister that I didn't trust myself or I didn't trust them. I went out on field service with married and single sisters. I worked with everyone from kids to the elderly.
I never had opposite sex friends until I quit being a JW. When I remarried I had a female friend (Kathy) be a grooms-person at my wedding. I see Kathy nearly everyday, I work with her now and occasionally we have lunch together. I have other opposite sex friends who I see less frequently. I don't see it as an issue at all. However, I think the Watchtower is preoccupied with sex and their doctrine is no one is trust worthy.
Thirdson