The senior members of this forum know what they are talking about. You do NOT have to get married if you don't want to. The only benefit this would serve is to expedite the process, since you wouldn't have to wait 12 months to then apply and then wait some more. Applying as common-law is perfectly acceptable as long as you can prove you are in a marriage-like relationship and many many people on this forum have had their visitor visa extended even several times on the grounds that they were wanting to establish common law to apply for Spousal PR.