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It is more common for men to keep on their tee shirts rather than their underwear. It is usually a sign of embarrassment about physique. One approach is to make love with a small candle burning which allows the intimacy, but avoids the embarrassment. I would also suggest you talk to your lover about it. Communications is so important in good sex which most people overlook. Best wishes.
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Talking, sufficient foreplay and ultimately erectile meds like Viagra or Cialysis should solve the problem.