The IRB has addressed this issue in a couple of Clarifications (2006) and one Law change (2012).
Briefly, tights are not appropriate wear for rugby because they negatively affect the ability of a tackler to grasp the legs of a ball carrier. People have tried to get around this by claiming tights are “underwear” or “support”. The Clarifications have made it very straightforward – no tights.
Tights are not “underwear”
In the Law on Players’ Clothing (4), it is clearly stated that players wear “…a jersey, shorts and underwear, and socks and boots.”
In 2006 the French asked for definitions. The relevant section in the IRB’s response is...
3. Underwear: an undergarment, that covers the body from the waist, having short or no legs but does end above the knees, and worn next to the skin or under clothing, and not attached to the jersey or shorts.
This makes tights not an allowed piece of “underwear”.
Tights are not “support”
In the Law on Players’ Clothing players are allowed to wear “…supports made of elasticized or compressible material which must be washable.”
Also in 2006 South Africa asked about a long-sleeved undergarment which extended past the jersey’s sleeves. This was response from the IRB was that...
the wearing of elastic ‘long sleeves’ is not a support as described in Law 4.1(a).
This also applies to tights.
Women Wearing Tights
In 2012 a change was made to Law 4.2 Special Additional Items for Women (trial variation)
4.2(b) Female players may wear cotton blend long tights with single inside leg seam under their shorts and socks.
Why is this for female players only?
The only reason the IRB allows tights for women (and not men) is religious. Many Muslim women follow hijab, which is the requirement that they are completely clothed when in public. This trial Law allows them to meet their religious needs while affording them the opportunity to play rugby.