not sure why, part two specifically says this
"5 Marriage according to religious rites: no compulsion to solemnize etc
(1)A person may not be compelled to—
(a)undertake an opt-in activity, or
(b)refrain from undertaking an opt-out activity.
(2)A person may not be compelled—
(a)10 to conduct a relevant marriage,
(b)to be present at, carry out, or otherwise participate in, a relevant
marriage, or
(c)to consent to a relevant marriage being conducted,where the reason for the person not doing that thing is that the relevant
15marriage concerns a same sex couple.
(3)In this section—
“opt-in activity” means an activity of the kind specified in an entry in the
first column of the following table which falls to be undertaken for the
purposes of any enactment specified in the corresponding entry in the
20second column;
“opt-out activity” means an activity which reverses, or otherwise
modifies, the effect of an opt-in activity."
the link
http://www.publications.parliament.uk/pa/bills/cbill/2013-2014/0003/cbill_2013-20140003_en_2.htm#pt1-pb2-l1g2
and the Court of Human Rights seems to have agreed that the bill as it stands is acceptable under human rights law including the opt out clause
"EUROPEAN CONVENTION ON HUMAN RIGHTS
Secretary Maria Miller has made the following statement under section 19(1)(a) of the
Human Rights Act 1998:
In my view the provisions of the Marriage (Same Sex Couples) Bill are compatible
with the Convention rights."