Religious leaders represent the beliefs (and hence opinions) of
their religious groups (followers).
As citizens, they are entitled to their opinion, just like you and
I.
In Singapore, when they state their beliefs and views, they are not
saying that the Govt must follow their religious beliefs. They are
stating their and the followers of that religion's beliefs (or
opinions).
As such, I don't see why we should exclude their opinion from
consideration when they are also fellow citizens like us.
This doesn't mean we are mixing politics with religion.