How ethical is it to have a guy as a best friend after marriage?

Lol...

It means yes or no

I think "ethical" is a relative term. So if you are ok with ur husband having a girl as his best friend then it may be ethical for u to have a guy as ur best friend. But ur husband's opinion also matters.

Fair as it sound

जीवनका कतिपय क्षणहरूमा नाप तौल गर्नु पर्ने हुन्छ I भन्छन केटा र केटिमा केवल मित्रता हुन सक्दैन I मायाको मुना पलाउन कति बेर लाग्छ र? तर कुनै पनि सम्बन्धको आधार 'प्रेम' नै हो I प्रेम बिना कुनै पनि सम्बन्ध सम्भव नै छैन I तर प्रेमका पनि प्रकार हुन्छन I आमाले आफ्नो सन्तानलाई दिने प्रेम, श्रीमतीले आफ्नो श्रीमानलाई दिने प्रेम, प्रेमिकाले आफ्नो प्रेमीलाई दिने प्रेम, बुबाले आफ्नो सन्तानलाई दिने प्रेम, साथीले आफ्नो साथीलाई दिने प्रेम, गुरुले आफ्नो शिस्यलाई दिने प्रेम, आदि I यसैकारण तिमीले आफ़्नोलागि आफ्नो उचित धर्म छुट्याउनु पर्दछ I जस्तै, एउटा क्षत्रियले आफ्नो धर्म छोडेर देशको रक्षा गर्दैन भने त्यो क्षत्रिय रहदैन I यदि तिमीले आफ्नो पत्नी धर्म को पालना नगरी पराया पुरुष संग हाँस-खेल गर्दै जीन्दगी बितौछौ भने तिम्रो र तिम्रो श्रीमानको सम्बन्धमा दरार आउन सक्दछ I यसै कारण, तिमीले तिम्रो र तिम्रो पुरुष मित्रको बिचमा एउटा रेखा खिच्नु आवस्यक छ, अन्यथा बैबाहिक जीवन उथल-पुथल हुन सक्छ I र आर्को कुरा, यदि तिमीले आफ्नो पत्नी धर्म पूर्ण रूपले पालना गरेकी छौ र पुरुष मित्रसंग मित्रताको सिमा उल्लंग्घन गरेकी छौनौ; र पनि तिम्रो श्रीमान तिम्रो उचित क्रियाकलाप प्रति असन्तुस्ट छन् भने, तिम्रो श्रीमान तिम्रो लायक छैनन् I

My personal opinion only: Disregard if you don't think the same way.
Having a guy as a best friend after marriage is of course ethical if you're a guy. :P
well, if you're a girl then as well it's not unethical.
However, in a good relationship, a husband/wife should be a best friend of each other to share everything between each other (else the relation would be just a formality). when husband/wife is a best friend of each other then you ain't need any other best friend no matter a guy or a girl.
I ain't saying a husband or a wife couldn't have a friend. They have professional life too. In work, in college/school friends are there who are always a best buddy however like ujl swami mentioned, there is a limitation. Anything over the limitation is no fair for anything.
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whatever I have said above only applies theoretically in case of me.
I would for sure trust my wife that's how a good relationship is formed however I would never want/let my wife have a guy best friend. Friend ok. No best friend. :)