We don't know for certain that Ashe and Lexi will have any sex scenes together on the Kitten route. Some characters will only have sex scenes on certain routes.
It's pretty safe to assume that Ashe and Lexi will have sex on the Siren route, but Ashe will probably be the one doing the fucking, and Lexi will probably do most of the sucking.
If Lexi has any sex scenes on the Cuckquean route, it may be with someone other than Ashe, and Lexi might be on the receiving end in that route, as well. This is all speculation, of course.
Also, in the event that Ashe and Lexi have sex in the Kitten route, Lexi is plenty big enough to give Ashe pleasure if Ashe is on the receiving end. Lexi looks like she'll be at least 4 inches long when she's hard, maybe 4.5. The highest concentration of nerve endings in a pussy are in the first 2 inches, and the G-spot is typically no more than 3 inches deep.