By default, in new TBXs link lines are bezier curves and have no (link type). Once you use a link type, that document remembers the last used link type. So, in your TBX that appears stuck on ‘yes’, simply make a new link and whilst doing so select ‘untitled’ from the link type pop-up (the pop-up label actually says ‘*untitled’); Tinderbox will then remember that setting.
You might also do to read this page on the Links Inspector.
Link lines can be curved or straight but are set, per-link type. That setting then applies for all links of that type. So, having re-set your TBX - as above - to use ‘untitled’ links, open the Links Inspector and select the untitled link type and tick the ‘linear’ box. All existing links using this link type will now be drawn as straight lines as will any new links using that link type. For a new document, you will want to open the Links Inspector before adding content and set the line type for ‘untitled’ links before adding new content. I’m not aware of any way to pre-set the latter.
Tinderbox is a toolbox and we all use it different ways. However, if coming to Tinderbox from mind-mapping software, be open to the fact that Tinderbox maps are a much more expressive way of looking at your data. A classical mind map is just an outline with a single root node drawn in circular rather than linear form.
Maps without lines, using other display facets such as adjacency, or colour or shape can be clearer and expressive. They are more flexible as the lack of a line removes a sense of hierarchy. With that in mind, be aware you can toggle the visibility of all links of a given link type. Try turning off your ‘untitled’ link visibility to see how things look. For instance the green things probably don’t want to sit with the red things , but amongst the green things, the light green might cluster separately from the dark green ones. That’s a rather trite example, but using maps this way can really aide exploration of data where you don’t absolutely know the structure in advance. I think of it like doing a jigsaw where you don’t have the picture from the box top. some parts, like corner and edge pieces are obvious. Otherwise we need to sort on shape/colour/texture - though in a Tinderbox context those facets might textual rather than visual whether in $Text or other attributes. Tinderbox’s support for incremental formalisation is the thing that keeps it at the front of my software toolset all the time.