Is it normal to have an aa sponsor of the opposite sex?

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.
It's irrelevant; Whether your sponsor is the same or opposite sex, or if the sponsor's sex changes halfway through your recovery, makes zero difference. You both have the same relationship with alcohol - you've both had enough. That's all that matters.
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.