There were many reasons why he couldn't go.
"There were" is a really useful opening in some circumstances (I'm kinda fond of "It was" too). But with a stronger subject, we can make this more meaningful, and more clearly unified as "his" thought. Try:
Tom mentally listed some of the reasons he couldn't go: He didn't have any money. He couldn't take time off work.
"Tom" identifies the "actor" of the paragraph, adding to the unity because we know now that this paragraph is about Tom and his thoughts. "Mentally listed" puts this in his viewpoint, because only the POV character can mentally do anything (if it were just "Tom listed," it might be in someone else's POV, because he could be listing out loud).
For some reason, maybe alliteration, maybe just trimming, I feel like replacing "wasn't about to go" with "refused to go".
Her baby sister was getting married, and Miriam refused to go to the wedding without a man on her arm—and if not him, it could be some rent-a-stud from an escort service.
Now a mistake I see in paragraphing is to put at the end what ought to be the first sentence of the next paragraph. For example:
Tom mentally listed some of the reasons he couldn't go: He didn't have any money. He couldn't take time off work. But as he glanced over at Miriam, he realized that no reason was good enough. Her baby sister was getting married, and Miriam wasn't about to go to the wedding without a man on her arm—and if not him, it could be some rent-a-stud from an escort service. He and Miriam had met the first day of law school in their civil procedure class.
Notice that last sentence is about a new subject—no longer reasons to attend or not to attend the wedding, but how Tom and Miriam met. That should start a new paragraph about their meeting. Those of you who have an "ear" for the rhythm of paragraphs will hear the discordance there at the end. You might not know why, but you hear that the paragraph has gone on too long. Honor that instinct. Figure out what's wrong—what's sticking out of the roundness of the unified paragraph.
So don't put the "topic sentence" of one paragraph into another. Know what your paragraph is about, and unify around that.
Watch especially for this in descriptive passages. You know how some writing books tell you to use all five senses when you describe? Well, I have issues with that (your character's dominant perceptive mode should determine which sense is emphasized), but if you do want to do a full description, utilizing sight, sound, taste, touch, and smell, don't smush them altogether willy-nilly. Consider that your character is paying attention to one thing at a time. What comes first? Sight? So deal with sight first. Not enough for a full paragraph? Hmm. Are you really telling me you can't find three things to "see" at that moment? And then three things to "hear"? If you have a paragraph mostly about what is seen, but a sentence of "hearing" in the middle, you'll be breaking the unity of the description and disorienting the reader.
Just keep in mind -- paragraphs are units of meaning: unified and meaningful. Your reader is going to feel and hear the paragraph as a unit (if you do it right). Paragraphs should be tightly focused and carefully structured.
Oh, and you might have noticed that paragraphs are considerably shorter these days. Our attention span is shorter too. If you find your paragraphs going to half a page or more, then go back, analyze—what is the unit of meaning, and when does it change or shift? That's where to start a new paragraph.