Now, ACT are classical liberals, interested in formal rather than substantive freedom - but there is a clear reason (quite apart from any beliefs on the nature of marriage) for liberals of any stripe to vote against the bill: namely, that it licenses discrimination on the basis of marriage. While many classical liberals view limits on private discrimination as a restriction on freedom, and so would support some of the effects of s7 of the bill, there are still two very good reasons for them to oppose it: firstly, that the bill would license discrimination by the state - a no-no even to those who find widespread private discrimination acceptable; and secondly, that it does so in a completely one-sided manner, allowing discrimination in favour of marriage but not against it. The proponents of private discrimination at least favour a legal level playing field, and this bill doesn't provide one. The only "liberals" who would be in favour of it are bigots-in-liberal-clothing like Stephen Franks.
(Hat Tip: I See Red).