Enabling ASM on a https Virtual Server...?

Indeed, for that you will have to configure a clientside SSL profile and decrypt the traffic. The ASM module can not inspect encrypted traffic at L7.

If for whatever reason you have to route encrypted traffic to end-server, you can also configure both, a clientside and a serverside SSL profile which will help you retain a similar level of network security. In such setup, F5 will offload the SSL to process ASM security-checks, and re-encrypt any permitted traffic prior to routing the requests to backend servers.