It seems silly to have milking to be aliased to breast_milking when there is an equal opportunity for it to be penis_milking as well. Both concepts are typically called "milking," so it doesn't make sense for only one of them to receive the "milking" tag. Since this will leave the milking tag without a home, I would suggest these two options:

1. Imply both penis_milking and breast_milking to milking.
2. Alias milking to milking_(disambiguation), informing the uploader/tag editor to change it accordingly.

I definitely suggest the second one, as breast milking and penis milking are incredibly different and it might have the adverse affect of some people tagging "milking" by itself without either of the two related tags, but I wanted to include it just in case. The second idea allows for more clean tag usage (such as how every type of "cuff" isn't implied to one "cuff" tag).