How was Jeff okay with Barnabas dating Maggie?

I didn't know who Jeff Clark was until about 3/4 into the film because the character was never established. Jeff and Maggie never spend any time together alone and there is no reference to them being a couple.

We see Barnabas courting Maggie--taking her out to restaurants, inviting her over for an intimate meal, walking with her through the wilderness in daylight (after his transfusion treatment), etc. However, Maggie never mentions she's already in a relationship. Instead she's happy to go on dates with Barnabas and exclaims how must she enjoys spending time with him because of his child-like appreciation of nature and whatnot. Did Jeff and Maggie have an open relationship where they could see other people?

I didn't realize Jeff and Maggie were even an item until after she's bitten and drained, when he's sitting by her bedside and kisses her on the face. The same goes for the Carolyn-Todd relationship; I didn't realize Carolyn was in a relationship either until after she's dead, when Todd is referred to as her fiance. How was anyone supposed to know the two were engaged when Carolyn is hotly pursuing her relative (yuck) Barnabas!