Any official UCI races require short sleeves and cycling shorts with legs - it says in the rules.

I think a bra top and short shorts must be fine if it's a warm day and it's not a race. Sunscreen application is a necessity and not an activity I would normally place in the erotic section, and getting a helping hand on the back is necessary for me, at least until I really do take up that yoga class. The skimpiest outfit I've worn is probably the Castelli Melbourne dress with the accompanying shorts - they're really short, and even my hubby commented on there being rather a large proportion of flesh showing. For club training I always wear proper jerseys and shorts, normally the club attire.

When riding in a pack, being pushed about a little is quite common - it's probably the only gathering of people in which touching one anothers' bottoms is quite normal. When climbing I regularly receive a helping push either on my bottom or on my lower back - depends on the height of the rider who pushes - I'm tiny and so many of them don't reach down to my bum, but they normally get a better push the lower towards the seat they push. Even in girls' packs this is quite common, and if you watch TdF you can see that they quite regularly push each other, either to mark that they want another rider to move or to make him take a lead.