I can't seem to find any official information of which ports Warhammer 40,000: Space Marine uses. I own the game for the PS3 and need to forward the ports. Even though it works without forwarding anything, it takes between five and twenty minutes to get a match like this.
I had a similar issue, which was solved by setting my PS3 as the DMZ in the router (effectively opening all ports).
In regards to security, this should be fine, I've had my PS3 as the DMZ since I bought it and have not had, nor heard of any issues doing so.