交際: a couple's relationship and wider social interaction
Compare the couple who decided to marry after a long relationship with interactions expanded through work. Read romantic and social contexts separately, preserving the difference between a marriage decision and a completed wedding.
A couple deciding to marry
「二人は長い交際を経て結婚を決めた。」 (After a long relationship, the couple decided to marry.)
Here 交際 refers to the couple's relationship before their decision. The decision is reported, but the sentence does not say a wedding has already taken place.
Interactions expanded through work
「仕事を通じて他の地域の人々との交際が広がった。」 (Work expanded my interactions with people from other regions.)
The interactions are with people from other regions. Work broadens social contact in this scene; it is not a report of an increasing number of romantic partners.
Use the context to identify the relationship
A couple deciding to marry and people interacting through work have different relationship contexts. 交際 alone does not make every interaction romantic or marital.
The first example reports a decision, and the second reports expanded interactions. A long relationship does not by itself supply other facts about trust or a completed marriage.
Must 交際 expanding through work mean that the speaker has more romantic partners?
Show the answer and why
No. It concerns wider interactions with people from other regions. That social context differs from the couple's relationship in the marriage-decision example.