When three Colorado marijuana entrepreneurs made a presentation to 40 Silicon Valley investors last year, it was as though they were discussing something dirty.

“There was an uncomfortable chuckle in the room, kind of like you’re talking about porn,” says Fulton Connor, the Sand Hill Angels Inc. member who arranged the meeting. His fellow venture capitalists just couldn’t get past the fact that marijuana remains prohibited under federal law.

They’re way more chill now, he says. In May, Connor’s firm, which typically invests in mainstream startups like the taxi-hailing app Flywheel, steered $200,000 to Tradiv, an online marketplace for wholesale cannabis. “They understand it more as a business and the direction of the market,” Connor says. “And they see where it’s becoming more acceptable.”

The stigma surrounding the $7.2 billion marijuana industry is evaporating as Americans grow increasingly comfortable with the concept, more states allow medical and recreational use, and pot businesses start generating...