Preposition + -ing.

[Kaito-Hacker]

We're really looking forward _____ dinner with you tomorrow.

A. having
B. to have
C. to having

I thought it would be A. having since -ing is often followed by preposition despite a bit weird sound. However, the answer is C. to having, which sounds weirder.

This confused me, I thought all the verb following 'to' must be infinitive. Can someone explain why answer is C? Is there any other place this kind exception, to + -ing, would occur?

[Barb_D]

I look // I am looking forward to Xing.

I look forward to working with you.
I look forward to seeing you.
I'm looking forward to working with you.
I'm looking forward to working with you.

[Raymott]

We're really looking forward _____ dinner with you tomorrow.

A. having
B. to have
C. to having

I thought it would be A. having since -ing is often followed by preposition despite a bit weird sound. However, the answer is C. to having, which sounds weirder.

This confused me, I thought all the verb following 'to' must be infinitive. Can someone explain why answer is C? Is there any other place this kind exception, to + -ing, would occur?

"To look forward to" is a phrasal verb which just happens to end with 'to'.
"Having" is a gerund here, so it is not a verb for the purposes of the rule that you mention.
I (subject) like (verb) having dinner (object).
I (subject) look forward to (verb) having dinner (object).