My Only Job Offer Is Temporary — Would You Take It?

I'm about to graduate, but my only current offer is a summer internship at a very small firm (3 people total, including HR). They've made it clear it's temporary and not a path to full-time.

My concern is that many firms hire full-time roles during the summer, so I don't want to lock myself into this and miss out on other opportunities. During the interview, the owner also asked if I was interviewing elsewhere and said they want to avoid hiring someone who would leave for a full-time role.

I'm not the type to accept a job and then back out, so I'm stuck. Do I take the internship and risk missing full-time opportunities, or decline it and keep focusing on full-time applicationsh?

They want me to start as soon as possible, so I want to give them an answer by Monday.