The behavior you described is subjective and can vary greatly depending on cultural norms, individual personalities, and personal comfort levels. Some people, regardless of gender, may feel comfortable lounging in their underwear when they have a close and relaxed relationship with a friend. On the other hand, others might prefer not to engage in such behavior due to personal boundaries or cultural norms.
The acceptability of this kind of behavior may also depend on the specific context and the level of comfort and trust between friends. In some friendships, it might be entirely normal and accepted, while in others, it might be considered inappropriate.
Ultimately, what is considered acceptable and comfortable varies from person to person. It's crucial for individuals to respect each other's boundaries and communicate openly about what they feel comfortable with in any given situation. If both friends are comfortable with the idea, there is no inherent harm in it as long as it doesn't violate any social or cultural norms, and both parties are respectful of each other's boundaries.