Is that a chapel, or is that a performance space (like the one in TNG where musical performances and plays would take place) in which a lectern has been set up in order to hold a wedding? As I noted previously, there were no religious functionaries on the Enterprise, the Captain of the ship performed a civil service there. There are no Starfleet chaplains. Ships counselors perform the jobs traditionally assigned to chaplains in the present day Armed Forces.
Since there are no religious functionaries in the crew, and presumably not a ton of weddings that take place onboard ship (we've only seen a handful, and only the one in Balance of Terror on TOS took place in such a room), it seems to me far more likely that some kind of performance or briefing room was set up for the civil service, than that they have a room set aside unused except when a redshirt bites it and they happen to want a military funeral in space far away from said dead crewman's family and friends.