Recently, while out and about, I was asked by some friends having coffee and dessert while seated at an outdoor table on the patio of a local coffee spot: "What about sitting cross-legged, with your legs tucked up and feet on the seat?" Apparently, someone these friends knew had done this a bit earlier at this establishment and there had been a conversation about how "acceptable" this posture might be -- or not -- especially out in public.
Here's The EoK's short answer: Anytime, indoors or out, when we are seated in a chair, pulled up to a table, our feet belong on the floor, so we can lean over to eat or to drink. It's just unacceptable to put one knee up, with a foot on the seat of our chair...bend our knees and put two feet up on the seat...sit cross-legged on our chair -- or, any variation of this while seated at a table, be it a kitchen, dining, outdoor dining or cafe table, public or private.
Now, of course, there can exceptions made for only the most casual situations, for instance: if we are picnicking and sitting on a blanket, perhaps we'll feel comfortable and be able to eat and drink without making a (distasteful-to-others) mess, if we sit cross-legged while we enjoy ourselves, the meal and our companions; while seated on outdoor furniture not drawn up to a table, say poolside or just out relaxing under a tree, it's conceivable that we might sit cross-legged or maybe pull one knee up to drink a beverage, nibble a few snacks and converse.
Good taste and the consideration of others' comfort always is uppermost, so we also don't sit so that underwear or inappropriate areas of our anatomy (covered or not) are just "out there" awkwardly as an embarrassing display for others.