Not advised. There are no "rules" about this, and aa is a voluntary organization where individual groups really decide what they want to see in their own group. In general, one wants to avoid "13'th stepping, " which is sexual activity with other group members, and particularly a sponsor. Such relationships can be very damaging to the individual and their sobriety. In gay persons, opposite sex sponsors may work.
Intimacy. I agree w dr. Wartenburg. The relationship w a sponsor is by definition - emotionally intimate. Allowing for emotional intimacy means letting down one's guard & being vulnerable. This is the perfect setting for sexual/ romantic feelings to grow (depending on gender preference). When the relationship w a sponsor moves in that direction - it complicates the process & isn't helpful for sobriety.